verify: drop unnecessary check for nullid

In 1f64dad11884 (verify: filter messages about missing null manifests
(issue2900), 2011-07-13), we started ignoring nullid in the list of
manifest nodeids to check. Then, in 954b09a82a61 (verify: do not choke
on valid changelog without manifest, 2012-08-21), we stopped adding
nullid to the list to start with. So let's drop the left-over check
now.

mercurial/verify.py

@@ -235,8 +235,6 @@ class verifier(object):
         if self.havemf:
             for c, m in sorted([(c, m) for m in mflinkrevs
                         for c in mflinkrevs[m]]):
-                if m == nullid:
-                    continue
                 self.err(c, _("changeset refers to unknown manifest %s") %
                          short(m))