ARM: stm32: do not try to use a ramdisk in defconfig

The original stm32 defconfig file looks for a ./rootfs.cpio file,
but that breaks with

  ../scripts/gen_initramfs_list.sh: Cannot open './rootfs.cpio'

when trying to build this kernel. Anybody who relies on the
current behavior should change their build scripts to append
the initramfs manually, as this patch removes that default
in order to help autmatic testing.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>