Remove the `pull_mirror_branch_prefix` feature flag

What does this MR do?

This feature flag gates functionality that was added, but never used, to mirror branches with a prefix. The implementation has been disabled-by-default for a year without action; it's time to remove it.

Documentation was never written.

This is mostly a revert of !17368 (merged) , with a few changes since a lot of code has changed around it in the meantime. We don't remove the database column in this MR - that's left for the future.

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

Closes #257835 (closed)