Remove wait_async_cb workaround rust-lang/rust#65489

[todo]

Rust seems to have a bit of a weird bug around the interaction of DerefMut, Pin and FnMut. See rust-lang/rust#65489.

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SunriseOS/libuser/src/types.rs

Lines 201 to 211 in f5ac31a

	// TODO: Remove wait_async_cb workaround rust-lang/rust#65489
	// BODY: Rust seems to have a bit of a weird bug around the
	// BODY: interaction of DerefMut, Pin and FnMut. See
	// BODY: https://github.com/rust-lang/rust/issues/65489.
	let this = &mut *self;
	if let Some(s) = (this.f)() {
	core::task::Poll::Ready(s)
	} else {
	let _ = syscalls::clear_event(this.handle);
	this.registered_on = Some(cx.waker().clone());

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This issue was generated by todo based on a TODO comment in f5ac31a when #512 was merged. cc @roblabla.