dirstate: directly call the dirstatemap in `set_untracked`

This function is only called in two places: the deprecated "remove" method and
in the new `set_untracked` method.

So we simply inline the appropriate content in `set_untracked`, paving the way
to dropping the deprecated code and its associated function.

Differential Revision: https://phab.mercurial-scm.org/D11313

mercurial/dirstate.py

@@ -500,7 +500,9 b' class dirstate(object):'
             self._drop(filename)
             return True
         else:
-            self._remove(filename)
+            self._dirty = True
+            self._updatedfiles.add(filename)
+            self._map.removefile(filename, in_merge=self.in_merge)
             return True
 
     @requires_no_parents_change