mq: drop `_cacheabletip` usage

Strip have dedicated work around to solve the same problem, strip is even a
fraction faster without that thanks to simpler update process of the branchcache.

hgext/mq.py

@@ -3469,26 +3469,6 b' def reposetup(ui, repo):'
 
             return result
 
-        def _cacheabletip(self):
-            q = self.mq
-            cl = self.changelog
-            qbase = None
-            if not q.applied:
-                if getattr(self, '_committingpatch', False):
-                    # Committing a new patch, must be tip
-                    qbase = len(cl) - 1
-            else:
-                qbasenode = q.applied[0].node
-                try:
-                    qbase = self.unfiltered().changelog.rev(qbasenode)
-                except error.LookupError:
-                    self.ui.warn(_('mq status file refers to unknown node %s\n')
-                                 % short(qbasenode))
-            ret = super(mqrepo, self)._cacheabletip()
-            if qbase is not None:
-                ret = min(qbase - 1, ret)
-            return ret
-
     if repo.local():
         repo.__class__ = mqrepo
 

tests/test-mq-qpush-fail.t

@@ -61,7 +61,6 b' test corrupt status file'
   patch queue now empty
   $ cp .hg/patches/status.orig .hg/patches/status
   $ hg qpush
-  mq status file refers to unknown node * (glob)
   abort: working directory revision is not qtip
   [255]
   $ rm .hg/patches/status .hg/patches/status.orig