sgi/rd: drop kvm build script as buildroot supports this

For kvm build there are no separate requirements as the buildroot
filesystem is used that already supports kvmtool and other packages
required for kvm testing. So drop kvm build script. For any kvm
testing, build buildroot to get required boot images.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: Iecbf47f24c76ad75b3a9009b0dde9d3c04037bed