Remove an obsolete workaround for GRPC unavailable errors

What does this MR do?

In gitaly#1029 (closed) , a GRPC issue was detected that needed us to terminate unicorn and sidekiq processes if it was encountered. This bug has been fixed in GRPC and GitLab for at least 6 months, but the termination code remains.

I'm working on Sidekiq subprocess supervision in https://gitlab.com/gitlab-org/gitlab-ce/issues/40396, and spotted this as a case where sidekiq might shut itself down unexpectedly; my life is much easier if this simply never happens :D

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Related to #40396 (closed)