ALSA: hda: generic: Drop the old mic-mute LED hook

Now all users of the old snd_hda_gen_add_micmute_led() have been
converted to the new LED-classdev variant, and we can make it local,
and remove the unused hda_gen_spec.micmute_led.update callback field.

Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200618110842.27238-8-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>