dirstate: use `reset_state` to drop file in `update_file_p1`

The `dropfile` function is on its way out.

Differential Revision: https://phab.mercurial-scm.org/D11439

mercurial/dirstate.py

@@ -556,7 +556,8 b' class dirstate(object):'
             possibly_dirty = True
         elif not (p1_tracked or wc_tracked):
             # the file is no longer relevant to anyone
-            if self._map.dropfile(filename):
+            if self._map.get(filename) is not None:
+                self._map.reset_state(filename)
                 self._dirty = True
                 self._updatedfiles.add(filename)
         elif (not p1_tracked) and wc_tracked: