Remove extraneous process group agent test code (#60903)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60903
RPC tests using process group backend were disabled for CI internally / externally. This is removing the code for process group (only) tests. Faulty agent tests which also use process group will be in a later PR.
Test Plan: Imported from OSS
Reviewed By: jbschlosser, mrshenli
Differential Revision: D29440674
Pulled By: H-Huang
fbshipit-source-id: 4724c189a110ac821c3f4f6f1f8a5c98e057a2a4
