upgrade: remove unnecessary `is None` check...
upgrade: remove unnecessary `is None` check
`upgrade_engine.upgrade()` always return the `backuppath` value and there are
not early returns. Hence I don't see how `backuppath` can be None.
Adding extra unncessary safe checks hides unknown bugs. Hence removing it.
Differential Revision:
https://phab.mercurial-scm.org/D9675