The beep->mutex is no longer used since the drop of beep_mode=2.
Let's get rid of it.

Fixes: 0920c9b4 ("ALSA: hda - Remove beep_mode=2")
Link: https://lore.kernel.org/r/20240222153148.19691-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>