HID: apple: remove unused members from struct apple_sc_backlight

The backlight members in struct apple_sc_backlight are unused. They were
added in commit 9018eacb (HID: apple: Add support for keyboard
backlight on certain T2 Macs.), but never used.

As this is not a struct to communicate with the HW, remove these.

Found by https://github.com/jirislaby/clang-struct

.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Paul Pawlowski <paul@mrarm.io>
Cc: Aun-Ali Zaidi <admin@kodeit.net>
Cc: Aditya Garg <gargaditya08@live.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>