OMAP: RX51: Remove unused old omapfb stuff

RX51 uses the new DSS2 display driver, but the board file still
contained some code for the old omapfb driver. The old code can be
removed.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>