dell-led: remove GUID check from dell_micmute_led_set()

As dell_micmute_led_set() no longer uses the dell_wmi_perform_query()
method, which was removed in commit 0c41a08e ("dell-led: use
dell_smbios_send_request() for performing SMBIOS calls"), the
DELL_APP_GUID check is redundant and thus can be safely removed.

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>