subrepo: remove argument introduced by mistake in c19b9282d3a7

mercurial/subrepo.py

@@ -269,7 +269,7 b' class abstractsubrepo(object):'
         """
         raise NotImplementedError
 
-    def merge(self, state, overwrite=False):
+    def merge(self, state):
         """merge currently-saved state with the new state."""
         raise NotImplementedError