branchmap: remove the droppednodes logic

It was unused. note how it is only extended if the list is empty. So it's always
empty at the end.

We could try to fix that, however this would part of the code is to be removed
in the next changeset as we do not run `branchmap` on truncated repo since
`repoview` in 2.5.

mercurial/branchmap.py

@@ -203,15 +203,13 b' class branchcache(dict):'
         # branch that ceased to exist may not be in newbranches because
         # newbranches is the set of candidate heads, which when you strip the
         # last commit in a branch will be the parent branch.
-        droppednodes = []
         for branch in self.keys():
             nodes = [head for head in self[branch]
                      if cl.hasnode(head)]
             if not nodes:
-                droppednodes.extend(nodes)
                 del self[branch]
 
-        if ((not self.validfor(repo)) or (self.tipnode in droppednodes)):
+        if not self.validfor(repo):
             # cache key are not valid anymore
             self.tipnode = nullid
             self.tiprev = nullrev