buildvm-s390x-20/21: drop one s390x builder and add memory to another
    
    webkitgtk is unable to finish building on the current builders, so
    lets drop one and add memory to another. Will make 20 the only one in
    the heavybuilder channel for it. Hopefully this will get it to complete.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>