Drop build_all_rootfs.sh

The build_rootfs.sh script will soon require the user to specify a
kernel and ramdisk in order to work, so it doesn't make sense to have
a "build all" script without also providing/building all the kernels,
which is onerous.

Change-Id: I7277ceb3ab8dd2316576c001d2b2185e53bb5e5b