Fix 'delete room' admin api to work on incomplete rooms (#11523)

If, for some reason, we don't have the create event, we should still be able to
purge a room.

synapse/handlers/pagination.py

@@ -406,9 +406,6 @@ class PaginationHandler:
             force: set true to skip checking for joined users.
         """
         with await self.pagination_lock.write(room_id):
-            # check we know about the room
-            await self.store.get_room_version_id(room_id)
-
             # first check that we have no users in this room
             if not force:
                 joined = await self.store.is_host_joined(room_id, self._server_name)

synapse/handlers/room.py

@@ -1535,20 +1535,13 @@ class RoomShutdownHandler:
             await self.store.block_room(room_id, requester_user_id)
 
         if not await self.store.get_room(room_id):
-            if block:
-                # We allow you to block an unknown room.
-                return {
-                    "kicked_users": [],
-                    "failed_to_kick_users": [],
-                    "local_aliases": [],
-                    "new_room_id": None,
-                }
-            else:
-                # But if you don't want to preventatively block another room,
-                # this function can't do anything useful.
-                raise NotFoundError(
-                    "Cannot shut down room: unknown room id %s" % (room_id,)
-                )
+            # if we don't know about the room, there is nothing left to do.
+            return {
+                "kicked_users": [],
+                "failed_to_kick_users": [],
+                "local_aliases": [],
+                "new_room_id": None,
+            }
 
         if new_room_user_id is not None:
             if not self.hs.is_mine_id(new_room_user_id):

synapse/rest/admin/rooms.py

@@ -106,9 +106,6 @@ class RoomRestV2Servlet(RestServlet):
                 HTTPStatus.BAD_REQUEST, "%s is not a legal room ID" % (room_id,)
             )
 
-        if not await self._store.get_room(room_id):
-            raise NotFoundError("Unknown room id %s" % (room_id,))
-
         delete_id = self._pagination_handler.start_shutdown_and_purge_room(
             room_id=room_id,
             new_room_user_id=content.get("new_room_user_id"),