Drop application handler on run loop exit (#4149)

Calling the `Drop` impl of the user's `ApplicationHandler` is important on
iOS and Web, since they don't return from `EventLoop::run_app`.

And now that we reliably call `Drop`, the `ApplicationHandler::exited`
event/callback is unnecessary; using `Drop` composes much better (open files
etc. stored in the app state will be automatically flushed), and prevents
weirdness like attempting to create a new window while exiting.

src/platform_impl/linux/wayland/event_loop/mod.rs

@@ -203,11 +203,10 @@ impl EventLoop {
         if !self.exiting() {
             self.poll_events_with_timeout(timeout, &mut app);
         }
+
         if let Some(code) = self.exit_code() {
             self.loop_running = false;
 
-            app.exiting(&self.active_event_loop);
-
             PumpStatus::Exit(code)
         } else {
             PumpStatus::Continue

src/platform_impl/linux/x11/mod.rs

@@ -498,8 +498,6 @@ impl EventLoop {
         if let Some(code) = self.exit_code() {
             self.loop_running = false;
 
-            app.exiting(self.window_target());
-
             PumpStatus::Exit(code)
         } else {
             PumpStatus::Continue