ARM: OMAP2+: Remove legacy board-flash.c

Legacy board files in mach-omap2 used the helper functions
board_{nor,nand,onenand}_init() to initialize the flash
devices attached to the GPMC.

With Device Tree booting the initialization is handled by
the GPMC driver gpmc_probe_*_child() functions so this
code is not needed anymore now that OMAP2+ is DT-only.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>