Clean up orphaned LFS file references during GC

What does this MR do?

There's no automatic removal of LFS objects from GitLab projects at the moment, if they become orphaned.

#215098 (closed) will introduce a way to do this fully automatically, but in the meantime, we have a pressing need for people to be able to remove (often very large) LFS objects from their projects once they're no longer referenced by the repository.

GitGarbageCollectWorker is an obvious place to put this, and will result in LFS cleanup happening at the same time as main repository cleanup - including when we run the "project cleanup" steps documented here: https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Implements #223621 (closed)