atomic: no need to provide repo for image builds
    
    Since we pull from ostree repos it doesn't really make
    sense to provide a yum "repo" to the image build in pungi.
    Removing it since it could be confusing to have it there.
    
    (cherry picked from commit dd006c0e8b2d725d66347c5ed4447c86ebb1ad91)
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>