function delete_tags_with_no_posts(){
	$tags = get_tags( array('number' => 0,'hide_empty' => false));
	foreach ( $tags as $tag ) {
		$tag_id = $tag->term_id;
		$args = array( 'post_type' => 'post', 'posts_per_page'   => -1,
			'tax_query' => array(
				array(
					'taxonomy' => 'post_tag',
					'field' => 'id',
					'terms' => $tag_id
				)
			)
		);
		$postslist = get_posts( $args );
		$count_posts_with_this_tag = count($postslist);
		if ($count_posts_with_this_tag < 1 ){
			wp_delete_term( $tag_id, 'post_tag' );
		}
	}
}
add_action( 'wp_head', 'delete_tags_with_no_posts' );