audio/noaudio: convert to QOM lifecycle methods

Remove the legacy driver init/fini callbacks from the null audio backend.

Both no_audio_init() and no_audio_fini() were no-ops that performed
no real initialization or cleanup work.

Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

audio/noaudio.c

@@ -119,16 +119,6 @@ static void no_enable_in(HWVoiceIn *hw, bool enable)
     }
 }
 
-static void *no_audio_init(Audiodev *dev, Error **errp)
-{
-    return &no_audio_init;
-}
-
-static void no_audio_fini (void *opaque)
-{
-    (void) opaque;
-}
-
 static struct audio_pcm_ops no_pcm_ops = {
     .init_out = no_init_out,
     .fini_out = no_fini_out,
@@ -146,8 +136,6 @@ static struct audio_pcm_ops no_pcm_ops = {
 
 static struct audio_driver no_audio_driver = {
     .name           = "none",
-    .init           = no_audio_init,
-    .fini           = no_audio_fini,
     .pcm_ops        = &no_pcm_ops,
     .max_voices_out = INT_MAX,
     .max_voices_in  = INT_MAX,