ARM: OMAP2+: Remove legacy gpmc-nand.c

This code is no longer used and can be removed as we
are using device tree.

Removing this code also removes a dependency between
drivers/mtd and arch/arm/mach-omap2 making furhter driver
changes easier.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
[tony@atomide.com: removed from header too, updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>