revlog: remove _chunkbase since it is no longer used

This was introduced in 2011 for the lwcopy feature but never actually got used.
A similar hook can easily be reintroduced if needed in the future.

mercurial/revlog.py

@@ -883,9 +883,6 b' class revlog(object):'
 
         return l
 
-    def _chunkbase(self, rev):
-        return self._chunk(rev)
-
     def _chunkclear(self):
         self._chunkcache = (0, '')