video: backlight: remove unused header

Commit 9befe40f ("video: backlight:
support s6e8ax0 panel driver based on MIPI DSI") added s6e8ax0.h, but
no file includes it. That's probably a good thing, because it declares
an extern void function that is defined static int in s6e8ax0.c.
Besides, that function is also wrapped in the module_init() macro, which
should do everything needed to make that function available to the code
outside of s6e8ax0.c. This header can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Donghwa Lee <dh09.lee@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>