backend: pruner: simplify the pool logic
    
    Callbacks in apply_async() are unnecessary, we don't have to take care
    if run_createrepo() finishes correctly or not - we just log the failure
    and worker goes on..
    
    The arguments passed down to run_createrepo() were duplicated in "cmd".
    So construct the called command down in run_createrepo() instead.
    
    Drop the __del__() method which is not guaranteed to be called.
    
    Don't ignore the signal;  because we don't implement our own signal
    handler, we definitely want to deliver CTRL+C to all processes in this
    process group (including createrepo_c or prunerepo).