ARM: shmobile: Remove marzen_defconfig

Now when the Marzen legacy board code is removed also
get rid of the marzen_defconfig file. Existing users
shall move over to shmobile_defconfig.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>