Remove EventLoopError::AlreadyRunning

This is already prevented by the type-system, and as such it doesn't
make sense to have an error case for this.

src/platform_impl/macos/event_loop.rs

@@ -279,10 +279,6 @@ impl<T> EventLoop<T> {
     where
         F: FnMut(Event<T>, &RootWindowTarget),
     {
-        if self.delegate.is_running() {
-            return Err(EventLoopError::AlreadyRunning);
-        }
-
         let callback = map_user_event(callback, self.receiver.clone());
 
         // # Safety