releng: drop clean-ami's job
    
    This job was used to cleanup old ami's from fedimg.
    It then reported to a pagure project about what it did.
    However, it's token has been expired and it no longer needs to exist.
    Garbage collection of ami's should take place in the new
    cloud-image-uploader (well, it marks when things are eol).
    So, lets just remove this old and nolonger needed cron.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>