Kill MessageType::SHUTDOWN related logic in pg agent (#31270)
Summary:
https://github.com/pytorch/pytorch/pull/30330 got rid of the need to send a `MessageType::SHUTDOWN` message, so we can now remove the logic/utils for this type of message.
I think we can also delete the enum entry in the `enum MessageType`, but we may want to keep it in case the logic in https://github.com/pytorch/pytorch/pull/30710 is ever moved to C++.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31270
Test Plan: All existing unit tests pass
Differential Revision: D19146983
Pulled By: rohan-varma
fbshipit-source-id: 35b185411f9446d7d4dfc37a6cb5477cf041e647
