omapfb: Remove unused writeback code

Remove unused writeback code. This code will never be used, as omapfb is
being deprecated.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313122410.7528-1-tomi.valkeinen@ti.com