setup.py: Remove conflicting double documentation of USE_FBGEMM (#36993)
Summary:
Line 33+ contains instructions on how to disable use, 108+ on how to enable it.
The default in CMakeLists.txt is enabled, so drop the latter.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36993
Differential Revision: D21161793
Pulled By: ngimel
fbshipit-source-id: 08c5eecaf8768491f90d4a52c338ecea32a0c35e
