Rip out auth-event reconciliation code (#12943)

There is a corner in `_check_event_auth` (long known as "the weird corner") where, if we get an event with auth_events which don't match those we were expecting, we attempt to resolve the diffence between our state and the remote's with a state resolution.

This isn't specced, and there's general agreement we shouldn't be doing it.

However, it turns out that the faster-joins code was relying on it, so we need to introduce something similar (but rather simpler) for that.

synapse/state/__init__.py

@@ -24,7 +24,6 @@ from typing import (
     DefaultDict,
     Dict,
     FrozenSet,
-    Iterable,
     List,
     Mapping,
     Optional,
@@ -422,31 +421,6 @@ class StateHandler:
         )
         return result
 
-    async def resolve_events(
-        self,
-        room_version: str,
-        state_sets: Collection[Iterable[EventBase]],
-        event: EventBase,
-    ) -> StateMap[EventBase]:
-        logger.info(
-            "Resolving state for %s with %d groups", event.room_id, len(state_sets)
-        )
-        state_set_ids = [
-            {(ev.type, ev.state_key): ev.event_id for ev in st} for st in state_sets
-        ]
-
-        state_map = {ev.event_id: ev for st in state_sets for ev in st}
-
-        new_state = await self._state_resolution_handler.resolve_events_with_store(
-            event.room_id,
-            room_version,
-            state_set_ids,
-            event_map=state_map,
-            state_res_store=StateResolutionStore(self.store),
-        )
-
-        return {key: state_map[ev_id] for key, ev_id in new_state.items()}
-
     async def update_current_state(self, room_id: str) -> None:
         """Recalculates the current state for a room, and persists it.