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

mercurial/upgrade.py

@@ -205,7 +205,7 b' def upgraderepo('
                 backuppath = upgrade_engine.upgrade(
                     ui, repo, dstrepo, upgrade_op
                 )
-            if not (backup or backuppath is None):
+            if not backup:
                 ui.status(
                     _(b'removing old repository content %s\n') % backuppath
                 )