Fix leak when using audio samples instead of streams

[adamscott]

Fixes #96068

The issue was caused by the fact that the Ref<AudioStreamPlayback> deletion from AudioServer::playback_list was done in the AudioServer::_mix_step() function. When a stream was replaced, it was tagged to be faded out. And when the fading was done, it was promptly deleted/unreferenced.

The thing is that samples never trigger _mix_step(), thus never clearing those references.

I now implemented the deletion in a function that _mix_step() calls and that AudioServer::stop_playback_stream() calls too. The later calls it if the stream is stopped, flagged as containing a sample, and that it's reference to the sample is nullptr.

[akien-mga]

Thanks!