function delete_empty_tags(){
	$tags = get_tags( array('number' => 0,'hide_empty' => false));
	foreach ( $tags as $tag ) {
		$tag_count = $tag->count;
		if ($tag_count < 1 ){
                   wp_delete_term( $tag->term_id, 'post_tag' );
		}
	}
}
add_action( 'wp_head', 'delete_empty_tags' );