fix: undprecate entity factory helpers (#101)

Closes #100
diff --git a/google/api_core/iam.py b/google/api_core/iam.py
index fc35469..59e5387 100644
--- a/google/api_core/iam.py
+++ b/google/api_core/iam.py
@@ -74,9 +74,6 @@
 _ASSIGNMENT_DEPRECATED_MSG = """\
 Assigning to '{}' is deprecated. Use the `policy.bindings` property to modify bindings instead."""
 
-_FACTORY_DEPRECATED_MSG = """\
-Factory method {0} is deprecated. Replace with '{0}'."""
-
 _DICT_ACCESS_MSG = """\
 Dict access is not supported on policies with version > 1 or with conditional bindings."""
 
@@ -323,12 +320,7 @@
 
         Returns:
             str: A member string corresponding to the given user.
-
-        DEPRECATED:  set the role `user:{email}` in the binding instead.
         """
-        warnings.warn(
-            _FACTORY_DEPRECATED_MSG.format("user:{email}"), DeprecationWarning,
-        )
         return "user:%s" % (email,)
 
     @staticmethod
@@ -341,12 +333,7 @@
         Returns:
             str: A member string corresponding to the given service account.
 
-        DEPRECATED:  set the role `serviceAccount:{email}` in the binding instead.
         """
-        warnings.warn(
-            _FACTORY_DEPRECATED_MSG.format("serviceAccount:{email}"),
-            DeprecationWarning,
-        )
         return "serviceAccount:%s" % (email,)
 
     @staticmethod
@@ -358,12 +345,7 @@
 
         Returns:
             str: A member string corresponding to the given group.
-
-        DEPRECATED:  set the role `group:{email}` in the binding instead.
         """
-        warnings.warn(
-            _FACTORY_DEPRECATED_MSG.format("group:{email}"), DeprecationWarning,
-        )
         return "group:%s" % (email,)
 
     @staticmethod
@@ -375,12 +357,7 @@
 
         Returns:
             str: A member string corresponding to the given domain.
-
-        DEPRECATED:  set the role `domain:{email}` in the binding instead.
         """
-        warnings.warn(
-            _FACTORY_DEPRECATED_MSG.format("domain:{email}"), DeprecationWarning,
-        )
         return "domain:%s" % (domain,)
 
     @staticmethod
@@ -389,12 +366,7 @@
 
         Returns:
             str: A member string representing all users.
-
-        DEPRECATED:  set the role `allUsers` in the binding instead.
         """
-        warnings.warn(
-            _FACTORY_DEPRECATED_MSG.format("allUsers"), DeprecationWarning,
-        )
         return "allUsers"
 
     @staticmethod
@@ -403,12 +375,7 @@
 
         Returns:
             str: A member string representing all authenticated users.
-
-        DEPRECATED:  set the role `allAuthenticatedUsers` in the binding instead.
         """
-        warnings.warn(
-            _FACTORY_DEPRECATED_MSG.format("allAuthenticatedUsers"), DeprecationWarning,
-        )
         return "allAuthenticatedUsers"
 
     @classmethod
diff --git a/tests/unit/test_iam.py b/tests/unit/test_iam.py
index e783533..fbd242e 100644
--- a/tests/unit/test_iam.py
+++ b/tests/unit/test_iam.py
@@ -230,72 +230,36 @@
         assert policy[VIEWER_ROLE] == expected
 
     def test_user(self):
-        import warnings
-
         EMAIL = "phred@example.com"
         MEMBER = "user:%s" % (EMAIL,)
         policy = self._make_one()
-        with warnings.catch_warnings(record=True) as warned:
-            assert policy.user(EMAIL) == MEMBER
-
-        (warning,) = warned
-        assert warning.category is DeprecationWarning
+        assert policy.user(EMAIL) == MEMBER
 
     def test_service_account(self):
-        import warnings
-
         EMAIL = "phred@example.com"
         MEMBER = "serviceAccount:%s" % (EMAIL,)
         policy = self._make_one()
-        with warnings.catch_warnings(record=True) as warned:
-            assert policy.service_account(EMAIL) == MEMBER
-
-        (warning,) = warned
-        assert warning.category is DeprecationWarning
+        assert policy.service_account(EMAIL) == MEMBER
 
     def test_group(self):
-        import warnings
-
         EMAIL = "phred@example.com"
         MEMBER = "group:%s" % (EMAIL,)
         policy = self._make_one()
-        with warnings.catch_warnings(record=True) as warned:
-            assert policy.group(EMAIL) == MEMBER
-
-        (warning,) = warned
-        assert warning.category is DeprecationWarning
+        assert policy.group(EMAIL) == MEMBER
 
     def test_domain(self):
-        import warnings
-
         DOMAIN = "example.com"
         MEMBER = "domain:%s" % (DOMAIN,)
         policy = self._make_one()
-        with warnings.catch_warnings(record=True) as warned:
-            assert policy.domain(DOMAIN) == MEMBER
-
-        (warning,) = warned
-        assert warning.category is DeprecationWarning
+        assert policy.domain(DOMAIN) == MEMBER
 
     def test_all_users(self):
-        import warnings
-
         policy = self._make_one()
-        with warnings.catch_warnings(record=True) as warned:
-            assert policy.all_users() == "allUsers"
-
-        (warning,) = warned
-        assert warning.category is DeprecationWarning
+        assert policy.all_users() == "allUsers"
 
     def test_authenticated_users(self):
-        import warnings
-
         policy = self._make_one()
-        with warnings.catch_warnings(record=True) as warned:
-            assert policy.authenticated_users() == "allAuthenticatedUsers"
-
-        (warning,) = warned
-        assert warning.category is DeprecationWarning
+        assert policy.authenticated_users() == "allAuthenticatedUsers"
 
     def test_from_api_repr_only_etag(self):
         empty = frozenset()