Remove unused "event_type" param on state.get_current_state_ids

this param doesn't seem to be used, and is a bit pointless anyway because it
can easily be replicated by the caller. It is also horrible, because it changes
the return type of the method.

synapse/state.py

@@ -146,8 +146,7 @@ class StateHandler(object):
         defer.returnValue(state)
 
     @defer.inlineCallbacks
-    def get_current_state_ids(self, room_id, event_type=None, state_key="",
-                              latest_event_ids=None):
+    def get_current_state_ids(self, room_id, latest_event_ids=None):
         if not latest_event_ids:
             latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
 
@@ -155,10 +154,6 @@ class StateHandler(object):
         ret = yield self.resolve_state_groups(room_id, latest_event_ids)
         state = ret.state
 
-        if event_type:
-            defer.returnValue(state.get((event_type, state_key)))
-            return
-
         defer.returnValue(state)
 
     @defer.inlineCallbacks