spanset: remove `.set()` definition

All my friends are dead.

mercurial/revset.py

@@ -2906,9 +2906,6 b' class _spanset(abstractsmartset):'
         l = baseset([r for r in self])
         return l[x]
 
-    def set(self):
-        return self
-
     def isascending(self):
         return self._start <= self._end