event_loop: remove generic user event

Let the users wake up the event loop and then they could poll their
user sources.

Co-authored-by: Mads Marquart <mads@marquart.dk>
Co-authored-by: daxpedda <daxpedda@gmail.com>

examples/child_window.rs

@@ -85,7 +85,7 @@ fn main() -> Result<(), impl std::error::Error> {
         event_loop.create_window(window_attributes).unwrap()
     }
 
-    let event_loop: EventLoop<()> = EventLoop::new().unwrap();
+    let event_loop = EventLoop::new().unwrap();
     let mut app = Application::default();
     event_loop.run_app(&mut app)
 }