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.

Author: tseaver

google/api_core/gapic_v2/__init__.py

@@ -94,18 +94,6 @@ def metadata(self):
         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

google/api_core/gapic_v2/dispatch.py

@@ -221,11 +221,3 @@ def test_from_gapic():
     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)