localrepo: remove spurious updatebranchcache return value

- it was left over from the refactoring in d01e28657429.

mercurial/localrepo.py

@@ -480,7 +480,7 b' class localrepository(repo.repository):'
     def updatebranchcache(self):
         tip = self.changelog.tip()
         if self._branchcache is not None and self._branchcachetip == tip:
-            return self._branchcache
+            return
 
         oldtip = self._branchcachetip
         self._branchcachetip = tip