dell-led: remove code related to mic mute LED

With dell_micmute_led_set() moved to drivers/platform/x86/dell-laptop.c,
all remnants of the mic mute LED handling code can be removed from
drivers/leds/dell-led.c, restoring it back to the state it was in before
commit db6d8cc0 ("dell-led: add mic mute led interface").

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Tested-by: Alex Hung <alex.hung@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>