Remove a missing import (#4821)

Works around #4815, although a proper fix is needed.

Author: srittau

third_party/2and3/google/protobuf/util/json_format_proto3_pb2.pyi

@@ -1,4 +1,7 @@
 # @generated by generate_proto_mypy_stubs.py.  Do not edit!
+
+# Manually edited to work around #4815.
+
 import sys
 from google.protobuf.any_pb2 import (
     Any as google___protobuf___any_pb2___Any,
@@ -42,9 +45,12 @@ from google.protobuf.timestamp_pb2 import (
     Timestamp as google___protobuf___timestamp_pb2___Timestamp,
 )
 
-from google.protobuf.unittest_pb2 import (
-    TestAllExtensions as google___protobuf___unittest_pb2___TestAllExtensions,
-)
+# Work-around #4815
+# from google.protobuf.unittest_pb2 import (
+#     TestAllExtensions as google___protobuf___unittest_pb2___TestAllExtensions,
+# )
+from typing import Any
+google___protobuf___unittest_pb2___TestAllExtensions = Any
 
 from google.protobuf.wrappers_pb2 import (
     BoolValue as google___protobuf___wrappers_pb2___BoolValue,