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
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors