branchcache: drop the unused `_verifyclosed`

This code appears dead since its introduction about 5 years ago in this three
consecutive commits:

- 6578654916ae → introduce the method with two calls
- 7c9d4cf23adf → remove first call
- be5eeaf5c24a → remove second call


    o  changeset:   be5eeaf5c24a
    |  user:        Pulkit Goyal <[e-mail]>
    |  date:        Fri Apr 05 15:57:09 2019 +0300
    |  summary:     branchcache: don't verify closed nodes in _branchtip()
    |
    o  changeset:   7c9d4cf23adf
    |  user:        Pulkit Goyal <[e-mail]>
    |  date:        Fri Apr 05 15:56:33 2019 +0300
    |  summary:     branchcache: don't verify closed nodes in iteropen()
    |
    o  changeset:   6578654916ae
    |  user:        Pulkit Goyal <[e-mail]>
    ~  date:        Mon Apr 01 13:56:47 2019 +0300
       summary:     branchcache: lazily validate nodes from the branchmap

mercurial/branchmap.py

@@ -438,8 +438,6 b' class branchcache(_BaseBranchCache):'
 
         # Do we need to verify branch at all ?
         self._verify_node = verify_node
-        # whether closed nodes are verified or not
-        self._closedverified = False
         # branches for which nodes are verified
         self._verifiedbranches = set()
         self._hasnode = None
@@ -553,7 +551,6 b' class branchcache(_BaseBranchCache):'
         # to hurt to overschedule
         other._delayed = self._delayed
         # also copy information about the current verification state
-        other._closedverified = self._closedverified
         other._verifiedbranches = set(self._verifiedbranches)
         return other
 
@@ -601,19 +598,6 b' class branchcache(_BaseBranchCache):'
                 % stringutil.forcebytestr(inst)
             )
 
-    def _verifyclosed(self):
-        """verify the closed nodes we have"""
-        if not self._verify_node:
-            return
-        if self._closedverified:
-            return
-        assert self._hasnode is not None
-        for node in self._closednodes:
-            if not self._hasnode(node):
-                _unknownnode(node)
-
-        self._closedverified = True
-
     def _verifybranch(self, branch):
         """verify head nodes for the given branch."""
         if not self._verify_node: