ARM: OMAP: remove "config FB_OMAP_CONSISTENT_DMA_SIZE"

The only user of Kconfig symbol FB_OMAP_CONSISTENT_DMA_SIZE got removed
in v3.8, with commit 6ba54ab4

 ("ARM:
OMAP: Remove omap_init_consistent_dma_size()"). Remove this symbol too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>