rpm_utils: remove is_installed()
    
    Due to a dnf 1.x bug (present on Fedora 25 and older), the previous
    implementation with `dnf list --installed` wasn't working properly - it
    also listed available packages, not just installed ones. Because a good
    fix is not simple and because this code is going away soon anyway, I
    removed the whole is_installed() method and no longer check installed
    packages. The packages are now always attempted to install (in
    production, under root, etc), or simply printed out as a warning on a
    developer setup. It's now up to the developer to notice the warning and
    install the packages if they're missing. Unfortunately it seems to be
    the most reasonable way to resolve this quickly.
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1220