Invalidate excuse after each policy run

This fixes commit 497edc to really allow policies to see if the excuse has
already been invalidated by previous policies.

britney.py

@@ -1616,7 +1616,6 @@ class Britney(object):
                 v = policy.apply_policy(policy_info, suite, src, source_t, source_u, excuse)
                 if v.value > policy_verdict.value:
                     policy_verdict = v
-
                     if policy_verdict.is_rejected:
                         excuse.is_valid = False