lcd: remove LCD_MONOCHROME

No one is using LCD_MONOCHROME; remove related code.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>