f-m: Remove qemu-user-static.

f-m: Remove qemu-user-static.

Previously, we added and used qemu-user-static and its wrapper
over debootstrap (qemu-debootstrap) in:
- requirements-rpm.txt
  No longer needed, qemu-user-static should be added automatically
  as a cobbler rpm dependency (similar to cobbler-grub-aarch64);
- prepare-build-env.sh
  No longer needed, we only support building packages natively.
  We never built an arm64 chroot at ISO build time.
- sandbox.mk
  Same motivation as above, no longer needed.
  I think this would have needed qemu-user-static on build machine,
  and binfmt properly set up on that machine as well, adding
  complexity to the build process, although we do not use it.

So, simplify things a little and prepare this for upstream.

Change-Id: Idfce5aa594f1fcd92b1f238c7d07cb88f26064b8
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>