remove unnecessary (hence misleading) rmi
remove unnecessary (hence misleading) rmi

A recently-added test included an unnecessary "rmi --prune".
Unnecessary code causes brain fatigue in maintainers trying
to understand its purpose. Kill it.

I've confirmed that tests pass under buildah root & rootless
and podman local & remote.

Signed-off-by: Ed Santiago <792d28329de7ed446c01b83f731a071248ffeaf4@redhat.com>

nalind/buildah-1,nalind/buildah-1,nalind/buildah-1