Delete duplicate copy of THCCachingAllocator. (#16226)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16226
Now that the caching allocator is moved to c10_cuda, we can
delete the duplicate copy from Caffe2.
Reviewed By: dzhulgakov, smessmer
Differential Revision: D13762540
fbshipit-source-id: 03f1ebf7f11c68c19aa0d66110156fe228da6138
