ARM: shmobile: Remove ARCH_SHMOBILE_MULTI

The migration from ARCH_SHMOBILE_MULTI to ARCH_RENESAS has been
completed in v4.12 by commit 9ed2d4bc ("ARM: dts: renesas:
Switch from ARCH_SHMOBILE_MULTI to ARCH_RENESAS"), so the
ARCH_SHMOBILE_MULTI symbol can be removed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>