sound: Do not set a recording source in mixer_uninit()

We currently set the recording source to SOUND_MIXER_MIC during mixer
deletion. Apart from the fact that this control might not be present on
all devices, it is unnecessary to do that, plus we already set all the
volumes to 0 in the mixer_set() call above.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D59076

sys/dev/sound/pcm/mixer.c

@@ -629,8 +629,6 @@ mixer_uninit(device_t dev)
 	for (i = 0; i < SOUND_MIXER_NRDEVICES; i++)
 		mixer_set(m, i, 0, 0);
 
-	mixer_setrecsrc(m, SOUND_MASK_MIC);
-
 	mtx_unlock(m->lock);
 
 	/* mixer uninit can sleep --hps */