Remove `Caffe2_MAIN_LIBS` (#38408)
Summary:
Right now it is an unused alias to `torch_library` interface library
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38408
Differential Revision: D21598250
Pulled By: malfet
fbshipit-source-id: ec9a2446b94e7ea68298831212005c2c80bbc95c
