drm: omapdrm: panel-lgphilips-lb035q02: Remove unused backlight GPIO

The backlight GPIO was supported with platform data only. Now that the
driver only supports DT, the backlight GPIO is never initialized. Remove
it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>