* Blacklist .proto source files is Bazel allows us to

This is a partial revert of https://github.com/protocolbuffers/protobuf/commit/7b28278c7d4f4175e70aef2f89d304696eb85ae3 to unblock, e.g., https://github.com/grpc/grpc/pull/21590 or https://github.com/lyft/envoy-mobile/issues/617 until Bazel is fixed.

Note: this is a forward-compatible change that automatically switches to the behavior intended by https://github.com/protocolbuffers/protobuf/commit/7b28278c7d4f4175e70aef2f89d304696eb85ae3 when a compatible Bazel is released without requiring users to upgrade Protobuf. We will revert this change when Bazel is fixed.

* Remove trailing ,

* Update BUILD