Remove legacy support for pickled Credentials.

Reviewed in http://codereview.appspot.com/5323055/.
diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index 3f2b727..1bc2aaa 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -75,7 +75,6 @@
   a two legged flow, and therefore has all of the required information to
   generate and refresh its own access tokens.
 
-  AssertionFlowCredentials objects may be safely pickled and unpickled.
   """
 
   def __init__(self, scope,
@@ -197,14 +196,10 @@
       return None
     if len(value) == 0:
       return None
-    credentials = None
     try:
       credentials = Credentials.new_from_json(value)
     except ValueError:
-      try:
-        credentials = pickle.loads(value)
-      except ValueError:
-        credentials = None
+      credentials = None
     return credentials
 
   def validate(self, value):
diff --git a/oauth2client/file.py b/oauth2client/file.py
index 60a6385..34a5f48 100644
--- a/oauth2client/file.py
+++ b/oauth2client/file.py
@@ -20,7 +20,6 @@
 
 __author__ = 'jcgregorio@google.com (Joe Gregorio)'
 
-import pickle
 import threading
 
 
@@ -74,18 +73,11 @@
     except IOError:
       return credentials
 
-    # First try reading as JSON, and if that fails fall back to pickle.
     try:
       credentials = Credentials.new_from_json(content)
       credentials.set_store(self)
     except ValueError:
-      # TODO(jcgregorio) On a future release remove this path to finally remove
-      # all pickle support.
-      try:
-        credentials = pickle.loads(content)
-        credentials.set_store(self)
-      except:
-        pass
+      pass
 
     return credentials
 
diff --git a/oauth2client/multistore_file.py b/oauth2client/multistore_file.py
index 02e36d1..cf43dd9 100644
--- a/oauth2client/multistore_file.py
+++ b/oauth2client/multistore_file.py
@@ -35,7 +35,6 @@
 import fcntl
 import logging
 import os
-import pickle
 import threading
 
 try:  # pragma: no cover
diff --git a/tests/test_oauth2client_file.py b/tests/test_oauth2client_file.py
index de5fcf2..2d6f3b5 100644
--- a/tests/test_oauth2client_file.py
+++ b/tests/test_oauth2client_file.py
@@ -91,12 +91,11 @@
     pickle.dump(credentials, f)
     f.close()
 
-    # Storage should be able to read that object.
-    # TODO(jcgregorio) This should fail once pickle support is removed.
+    # Storage should be not be able to read that object, as the capability to
+    # read and write credentials as pickled objects has been removed.
     s = Storage(FILENAME)
-    credentials = s.get()
-    self.assertNotEquals(None, credentials)
-    self.assertEquals('foo', credentials.access_token)
+    read_credentials = s.get()
+    self.assertEquals(None, read_credentials)
 
     # Now write it back out and confirm it has been rewritten as JSON
     s.put(credentials)