remove dnf cache support
    
    Optimizing with `dnf --cacheonly` was probably a mistake to begin with.
    Minions that fail to rebuild regularly (often Branched and Rawhide) do
    not get updated libtaskotron (since repos are not refreshed). The gain
    from dnf repo caching is also not large, because most tasks need to
    install at least one dependency, and therefore the repo metadata are
    retrieved a shortly after anyway.
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1221