ARM: shmobile: remove ARCH_SHMOBILE_LEGACY option

With Bock-W, the last legacy platform has been removed.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>