ARM: shmobile: Kconfig: Get rid of old comment

Remove comment that used to be part of non-multiplatform
kernel support. Now with multiplatform-only we have no
Renesas-specific System Configuration entries.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>