Remove unused header in process_group_agent.h (#35767)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/35767
Test Plan: Imported from OSS
Differential Revision: D20771900
Pulled By: mrshenli
fbshipit-source-id: af88abfabfbe3d2d94407942738f7dcdfc3f30e2
