Switch Arc<Mutex<Keymap>> to Rc<RefCell<Keymap>>, a relic of the GPUI2 port.

Make gpui pass clippy

crates/gpui/src/subscription.rs

@@ -41,7 +41,6 @@ where
     /// are inert, meaning that they won't be listed when calling `[SubscriberSet::remove]` or `[SubscriberSet::retain]`.
     /// This method returns a tuple of a [`Subscription`] and an `impl FnOnce`, and you can use the latter
     /// to activate the [`Subscription`].
-    #[must_use]
     pub fn insert(
         &self,
         emitter_key: EmitterKey,