ARM: shmobile: fix defconfig warning on CONFIG_USB

A recent update to the marzen_defconfig introduced a
duplicate CONFIG_USB=y line. This removes one of the
two.

arch/arm/configs/marzen_defconfig:86:warning: override: reassigning to symbol USB

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Cc: linux-sh@vger.kernel.org