Remove feature flags for UserUpdateBranch

What does this MR do?

Remove the last Gitaly feature flag, at this moment. The feature flag code cleanup will happen in a new MR later, as there are still a few RPCs which we're not sure about if they'll get a feature flag, but if so we don't want to reintroduce the code.

What are the relevant issue numbers?

Linked in the commit

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