Remove DEBUG macro support for old debug levels
    
    Remove support for specifying old debug levels to the DEBUG macro:
    
        * remove debug_get_level function which was used for conversion,
        * remove debug_get_level tests,
        * remove mentions of old/new levels from DEBUG and DEBUG_IS_SET
          macro descriptions,
        * rename "newlevel" argument of debug_fn to just "level".
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Stephen Gallagher <sgallagh@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>
    (cherry picked from commit 5c233380e1ebf641f6106a34d7b94f9e9a606589)