ALSA: hda: generic: Drop unused snd_hda_gen_fixup_micmute_led()

The fixup function is no longer used.  Let's drop.

Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200618110842.27238-7-tiwai@suse.de


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