Remove build variable NCCL_EXTERNAL (#22467)
Summary:
It's always set to equal USE_NCCL, we made Gloo depending on Caffe2 NCCL
build. See 30da84fbe1614138d6d9968c1475cb7dc459cd4b
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22467
Differential Revision: D16098581
Pulled By: ezyang
fbshipit-source-id: f706ec7cebc2e6315bafca013b669f5a72e04815
