[backend] move clean_copr from prunerepo to our codebase
    
    Taken from ea98222d703c1589b44e0e777062eba34dcc3974 (prunerepo.git)
    
    Obviously, some changes needed to be made. All `args.foo` parameters
    from prunerepo's commandline were substituted to `clean_copr(...)`
    parameters.