ALSA: hda - Remove beep_mode=2

The beep_mode=2 option was introduced to make the beep mixer
controlling the beep input allocation/deallocation dynamically, so
that a user can switch between HD-audio codec digital beep and the
system beep only via mixer API.  This was necessary because the
keyboard driver took only the first input beep instance at that time.

However, the recent keyboard driver already processes the multiple
input instances, thus there is no point to keep this mode.

Let's remove it.

Acked-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>