audio: Remove an assertion that no longer makes sense.

One may happen to call SDL_AudioThreadFinalize when thread_alive is not set.

src/audio/SDL_audio.c

@@ -663,7 +663,6 @@ void SDL_QuitAudio(void)
 
 void SDL_AudioThreadFinalize(SDL_AudioDevice *device)
 {
-    SDL_assert(SDL_AtomicGet(&device->thread_alive));
     if (SDL_AtomicGet(&device->condemned)) {
         if (device->thread) {
             SDL_DetachThread(device->thread);  // no one is waiting for us, just detach ourselves.