video: fbdev: remove unused auo_k190xfb drivers

auo_k1900fb and auo_k1901fb drivers have been introduced six
years ago by following commits:

commit 2c8304d3 ("video: auo_k190x: add code shared by controller drivers")
commit 96b1d500 ("video: auo_k190x: add driver for AUO-K1900 variant")
commit 53027cdf

 ("video: auo_k190x: add driver for AUO-K1901 variant")

They never had any in-kernel user so just remove them (since
they are platform drivers they need corresponding platform
devices to be registered by kernel and it has never happened).

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>