remove ostree pruning from nightly.sh
    
    We'll do that now in a more centralized manner now that we are
    moving to a unified ostree repo setup.
    
    See https://pagure.io/releng/pull-request/7366
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>
    (cherry picked from commit 3c489e6d910309d3760e14b57be12422568aed80)