Minor optimisation of force handling

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

britney.py

@@ -1499,7 +1499,7 @@ class Britney(object):
         forces = self.hints.search('force', package=src, version=source_u.version)
         if forces:
             excuse.dontinvalidate = True
-        if not excuse.is_valid and forces:
+            if not excuse.is_valid:
                 excuse.addhtml("Should ignore, but forced by %s" % (forces[0].user))
                 excuse.force()
                 excuse.is_valid = True