Back out changes from #6267 / 'api_core-1.6.0a1' release. (#6328)

We will bring them back in when we have an OK to make a non-alpha
'api_core-1.6.0' release.

Toward #6326.
diff --git a/google/api_core/gapic_v2/__init__.py b/google/api_core/gapic_v2/__init__.py
deleted file mode 100644
index 4a5cc70..0000000
--- a/google/api_core/gapic_v2/__init__.py
+++ /dev/null
@@ -1,27 +0,0 @@
-# Copyright 2018 Google LLC
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-from google.api_core.gapic_v1 import client_info
-from google.api_core.gapic_v1 import config
-from google.api_core.gapic_v1 import method
-from google.api_core.gapic_v1 import routing_header
-from google.api_core.gapic_v2 import dispatch
-
-__all__ = [
-    'client_info',
-    'config',
-    'dispatch',
-    'method',
-    'routing_header',
-]
diff --git a/google/api_core/gapic_v2/dispatch.py b/google/api_core/gapic_v2/dispatch.py
deleted file mode 100644
index 7b107a2..0000000
--- a/google/api_core/gapic_v2/dispatch.py
+++ /dev/null
@@ -1,37 +0,0 @@
-# Copyright 2018 Google LLC
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-import functools
-
-
-def dispatch(func):
-    """Return a decorated method that dispatches on the second argument.
-
-    This is the equivalent of :meth:`functools.singledispatch`, but for
-    bound methods.
-    """
-    base_dispatcher = functools.singledispatch(func)
-
-    # Define a wrapper function that works off args[1] instead of args[0].
-    # This is needed because we are overloading *methods*, and their first
-    # argument is always `self`.
-    @functools.wraps(base_dispatcher)
-    def wrapper(*args, **kwargs):
-        return base_dispatcher.dispatch(args[1].__class__)(*args, **kwargs)
-
-    # The register function is not changed, so let singledispatch do the work.
-    wrapper.register = base_dispatcher.register
-
-    # Done; return the decorated method.
-    return wrapper
diff --git a/google/api_core/operation.py b/google/api_core/operation.py
index c76ac78..51a7a96 100644
--- a/google/api_core/operation.py
+++ b/google/api_core/operation.py
@@ -94,18 +94,6 @@
         return protobuf_helpers.from_any_pb(
             self._metadata_type, self._operation.metadata)
 
-    @classmethod
-    def deserialize(self, payload):
-        """Deserialize a ``google.longrunning.Operation`` protocol buffer.
-
-        Args:
-            payload (bytes): A serialized operation protocol buffer.
-
-        Returns:
-            ~.operations_pb2.Operation: An Operation protobuf object.
-        """
-        return operations_pb2.Operation.FromString(payload)
-
     def _set_result_from_operation(self):
         """Set the result or exception from the operation if it is complete."""
         # This must be done in a lock to prevent the polling thread
diff --git a/tests/unit/gapic/test_dispatch.py b/tests/unit/gapic/test_dispatch.py
deleted file mode 100644
index aa7d871..0000000
--- a/tests/unit/gapic/test_dispatch.py
+++ /dev/null
@@ -1,36 +0,0 @@
-# Copyright 2018 Google LLC
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-import pytest
-import six
-
-from google.api_core.gapic_v2.dispatch import dispatch
-
-
-@pytest.mark.skipif(six.PY2, reason='dispatch only works on Python 3.')
-def test_dispatch():
-    class Foo(object):
-        @dispatch
-        def bar(self, number, letter):
-            return 'Brought by the letter {} and the number {}.'.format(
-                letter, number,
-            )
-
-        @bar.register(str)
-        def _bar_with_string(self, letter):
-            return self.bar(11, letter)
-
-    foo = Foo()
-    assert foo.bar(8, 'L') == 'Brought by the letter L and the number 8.'
-    assert foo.bar('Z') == 'Brought by the letter Z and the number 11.'
diff --git a/tests/unit/test_operation.py b/tests/unit/test_operation.py
index 240eb86..211fea6 100644
--- a/tests/unit/test_operation.py
+++ b/tests/unit/test_operation.py
@@ -221,11 +221,3 @@
     assert future._metadata_type == struct_pb2.Struct
     assert future.operation.name == TEST_OPERATION_NAME
     assert future.done
-
-
-def test_deserialize():
-    op = make_operation_proto(name='foobarbaz')
-    serialized = op.SerializeToString()
-    deserialized_op = operation.Operation.deserialize(serialized)
-    assert op.name == deserialized_op.name
-    assert type(op) is type(deserialized_op)