use only createrepo_c, drop createrepo
    
    createrepo_c/mergerepo_c already supports everything we needs in koji.
    There is no reason to support internal implementation of mergerepo and
    all distributions in sight already have createrepo_c.
    
    Commit is dropping all createrepo/mergerepo uses and replace them with
    their _c variants. Config option use_creterepo_c is deprecated and
    prints warning to kojid.log. It should be removed completely in koji
    1.21.
    
    Fixes: https://pagure.io/koji/issue/1336