Rename unused variable/return value to "_"

This makes it more explicit that we are not using the value for
anything (and its name did not match it content due to a mistake
in 5d49a41204).

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -2057,7 +2057,7 @@ class Britney(object):
 
             if init:
                 # init => a hint (e.g. "easy") - so do the hint run
-                (_, nuninst_end, undo_list,) = self.try_migration(selected,
+                (_, nuninst_end, _) = self.try_migration(selected,
                                                          self.nuninst_orig,
                                                          transaction,
                                                          stop_on_first_regression=False)