internal/mux: remove a hack to avoid goroutines for Windows

[hajimehoshi]

Windows timer resolution was not good and caused some issues (hajimehoshi/ebiten#1768), and mux.playerImpl.Play has a hack to avoid goroutines for Windows.

oto/internal/mux/mux.go

Lines 216 to 222 in 457cd3e

	// Goroutines don't work effiently on Windows. Avoid using them (hajimehoshi/ebiten#1768).
	if runtime.GOOS == "windows" {
	p.m.Lock()
	defer p.m.Unlock()
	p.playImpl()
	} else {

As of Go 1.23, Windows uses much better timer reoslutions golang/go#44343 so we might no longer need this hack in the future.