[ARM] pxa: removing dead BACKLIGHT_CORGI

BACKLIGHT_CORGI was later renamed to BACKLIGHT_GENERIC and has not been
used since then. As it's now safe to completely transition to LCD_CORGI
driver, which incorprates both the LCD and backlight device.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>