clfilter: remove the last usage of `visibleheads`

We can just inline the filtering now that all server code uses
changelog filtering directly.

mercurial/discovery.py

@@ -115,7 +115,7 b' def findcommonoutgoing(repo, other, only'
         og.missingheads = onlyheads or repo.heads()
     elif onlyheads is None:
         # use visible heads as it should be cached
-        og.missingheads = visibleheads(repo)
+        og.missingheads = repo.filtered("unserved").heads()
         og.excluded = [ctx.node() for ctx in repo.set('secret() or extinct()')]
     else:
         # compute common, missing and exclude secret stuff