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/android/mod.rs

@@ -546,8 +546,6 @@ impl EventLoop {
         if self.exiting() {
             self.loop_running = false;
 
-            app.exiting(&self.window_target);
-
             PumpStatus::Exit(0)
         } else {
             PumpStatus::Continue