clean up stale conditionals
    
    All but two of the `%if 0%{?fedora} >= 15 || 0%{?rhel} >= 7`
    conditionals were removed in 76c7fad (Fix systemd
    executions/requirements, 2018-01-25).  Remove the others.
    
    Remove Fedora version from `%if 0%{?fedora} > 27 || 0%{?rhel} > 7`.
    Fedora 27 has been EOL since 2018-11-30.