update-ref: Make using 'symref-update' default behavior
We added a featureflag to rollout using 'symref-update' in 'git-update-ref(1)'. The rollout is now done, so we can remove the flag and cleanup all code.
This involves cleaning the old flow where we manually obtained a lock on the 'HEAD' file and instead now relying completely on the reference transaction hook.
Closes #6391 (closed)