postfix / main.cf: fix releng and vpn config and drop phx2
    
    We fixed the config in a PR from aheath1992 for most of the machines,
    but we need to fix vpn (proxies in particular) and releng boxes now.
    Also, while we are here, lets drop the phx2 file since it's not used
    anymore.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>