Remove delegate support from GPUI

We added this because we thought it would save some allocations when
sending operations given that we could move them to the delegate upon
notifying it, but the reality is that we serialize operations and that
only requires a reference.

crates/project/src/project.rs

@@ -942,7 +942,7 @@ impl Project {
                 remote_id
             ))?,
         }
-        cx.become_delegate(buffer, |this, buffer, event, cx| {
+        cx.subscribe(buffer, |this, buffer, event, cx| {
             this.on_buffer_event(buffer, event, cx);
         })
         .detach();
@@ -1028,7 +1028,7 @@ impl Project {
     fn on_buffer_event(
         &mut self,
         buffer: ModelHandle<Buffer>,
-        event: BufferEvent,
+        event: &BufferEvent,
         cx: &mut ModelContext<Self>,
     ) -> Option<()> {
         match event {