overlayworkingctx: add a no-op ``clearunknown()``

We no longer inherit ``workingctx``'s version, but we also don't need to do
anything anymore.

Differential Revision: https://phab.mercurial-scm.org/D1239

mercurial/context.py

@@ -2217,6 +2217,9 b' class overlayworkingfilectx(workingfilec'
     def remove(self, ignoremissing=False):
         return self._parent.remove(self._path)
 
+    def clearunknown(self):
+        pass
+
 class workingcommitctx(workingctx):
     """A workingcommitctx object makes access to data related to
     the revision being committed convenient.