mtd: remove the get_unmapped_area method

It is now unused.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Richard Weinberger <richard@nod.at>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Richard Weinberger <richard@nod.at>