memctx: remove flags since it is now inherited

commitablectx has a much more robust implementation of flags() so we will use
that instead of just blindly calling the flags function for the given path.

mercurial/context.py

@@ -1578,9 +1578,6 class memctx(committablectx):
             self._text = editor(self._repo, self, [])
             self._repo.savecommitmessage(self._text)
 
-    def flags(self, f):
-        return self[f].flags()
-
     def parents(self):
         """return contexts for each parent changeset"""
         return self._parents