drop a FIXME on enabling/disabling repos with sed
    
    dnf5 config-manager can now do this, but the syntax is different
    to dnf4, and honestly, it seems easier to just stick with this
    going forward than make it conditional on dnf version until
    dnf4 goes away. So let's stop marking this as a FIXME.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>