Add a script to clean up potential left over side-tags
    
    If a run of monitor-gating fails, there can be some side-tags left
    over. While we need to fix monitor-gating to avoid them, this script
    can also become handy to just clean up when needed.
    
    Relates to #18
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>