pre_get_posts Remove tax_query Completely

This fixed it! 🙂 Hat-tip gmazzap for the useful info: Obliterate the main query and replace it

function wpse_286813_omit_all( $query_vars ){

  // triggered also in admin pages
  if ( is_admin() )
    return $query_vars;

  if( !empty($query_vars['taxo']) && $query_vars['taxo'] === 'all' ){
    $query_vars['taxo']='';
  }
  return $query_vars;

}
add_filter( 'request', 'wpse_286813_omit_all' );