clean up unnecessary c++ extern imports

Summary:
A lot of these imports with specialization is not necessary. Clean up them to use generic functions instead.

(Note: this ignores all push blocking failures!)

Reviewed By: yfeldblum

Differential Revision: D24236181

fbshipit-source-id: fe6e6c41f680561c625b263bc6adeba738ddb252

folly/python/memory.pxd

+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# 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 libcpp.memory cimport shared_ptr, unique_ptr
+
+cdef extern from "folly/Memory.h" namespace "folly" nogil:
+    shared_ptr[T] to_shared_ptr[T](unique_ptr[T]&& ptr)