rebase: remove now unnecessary logic to allow empty commit when branch changes
This was a workaround for a bug in the empty commit check in repo.commit(),
where the parent branch name was incorrectly compared with the wdir branch name
instead of the branch name passed via extra. The bug was fixed in D8724.
The workaround was introduced in b2415e94b2f5.
Differential Revision: https://phab.mercurial-scm.org/D8725