builders_stg: do not set staging as datacenter for staging builders
    
    This has caused them to pick up the iptables template for staging
    instead of the one for their host group (koji_builder). There's no
    reason to have staging as a datacenter for these, nothing checks it
    besides the base iptables role hopefully.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>