Remove obsolete gitlab_shell checks

What does this MR do?

Remove the broken git storage from the storage shards. This behaviour isn't required anymore, as the :broken_storage trait can be set on the context block. This doesn't make a Gitaly call at all.

In turn this MR is also required to merge gitaly!675 (merged) on the Gitaly project. Else it wouldn't boot for anyone.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

/cc @alejandro