Shoutbox

This is a feature of messenger itself. Go to Contacts > sort contacts by and choose "Status".

If you rather keep them in groups: keep Groups ticked and make sure you enable "group offline contacts together" is checked.

Edit: or the solution posted by duck above if you really do not want to see them anywhere. Note: this will not remove the group "Offline" (it will only be empty), this group can not be removed.