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/timeline.rs

@@ -662,7 +662,7 @@ impl Service {
         // Our depth is the maximum depth of prev_events + 1
         let depth = prev_events
             .iter()
-            .filter_map(|event_id| Some(services().rooms.timeline.get_pdu(event_id).ok()??.depth))
+            .filter_map(|event_id| Some(self.get_pdu(event_id).ok()??.depth))
             .max()
             .unwrap_or_else(|| uint!(0))
             + uint!(1);