Remove BUILD_ATEN_ONLY build option (#24441)
Summary:
This build option no longer works.
Close https://github.com/pytorch/pytorch/issues/21703
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24441
Differential Revision: D17138131
Pulled By: ezyang
fbshipit-source-id: 67adac990645a5df1f7c2e2dbef3689b2c30fcf8
