remove local aliases from old room on upgrade

Without explicitly removing the old alias when reassigning aliases, we
were listing the alias on both rooms in the GET client/v3/rooms/{roomId}/aliases
endpoint. Resolving the alias still always pointed to the correct room.

src/service/rooms/alias.rs

@@ -40,6 +40,9 @@ impl Service {
             ));
         }
 
+        if self.resolve_local_alias(alias)?.is_some() {
+            self.remove_alias(alias, user_id)?;
+        }
         self.db.set_alias(alias, room_id)
     }