enable unused_self lint

Functions using `services()` are allowed to pointlessly take `self`
because the existence of `services()` is a crime and the solution is
making the types store references to their dependencies and then going
through `self`, so just allowing the lint saves us from modifying some
code only to switch it back later. Much later. Getting rid of
`services()` will probably be an ordeal.

src/service/rooms/pdu_metadata.rs

@@ -40,7 +40,11 @@ impl Service {
         }
     }
 
-    #[allow(clippy::too_many_arguments)]
+    #[allow(
+        clippy::too_many_arguments,
+        // Allowed because this function uses `services()`
+        clippy::unused_self,
+    )]
     pub(crate) fn paginate_relations_with_filter(
         &self,
         sender_user: &UserId,