Remove protobuf dependency in pytorch cmake file. (#14182)

Remove protobuf dependency in pytorch cmake file. (#14182)

Summary:
Currently, pytorch doesn't dependent on protobuf. So, we don't need to include the protobuf dir in pytorch cmake file.
And if we build caffe2 without custom-protobuf[1], we will have the protobuf mismatched problem.

[1]
https://github.com/pytorch/pytorch/blob/92dbd0219f6fbdb1db105386386ccf92c0758e86/CMakeLists.txt#L65
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14182

Differential Revision: D13356273

Pulled By: ezyang

fbshipit-source-id: 8120c3452d158dc51d70156433d7b9076c6aed47