invariance is maintained by top-level EventLoopWindowTarget<T>

this field is transitional and her to keep API compatibility only.
the correct variance and such is already ensured by the top-level
`EventLoopWindowTarget`, just use `PhantomData<T>` here.

src/platform_impl/windows/event_loop.rs

@@ -219,10 +219,7 @@ pub struct EventLoopWindowTarget<T: 'static> {
     thread_id: u32,
     thread_msg_target: HWND,
     pub(crate) runner_shared: EventLoopRunnerShared<UserEventPlaceholder>,
-    // TODO
-    // eventually should be removed after all the backends refactored,
-    // but for now should this be invariant or contra-variant to T?
-    _marker: PhantomData<*mut T>,
+    _marker: PhantomData<T>,
 }
 
 impl<T: 'static> EventLoop<T> {