Add action on delete comments?

Look at the function wp_delete_comment(). It fires an action before the comment is deleted:

/**
 * Fires immediately before a comment is deleted from the database.
 *
 * @since 1.2.0
 *
 * @param int $comment_id The comment ID.
 */
do_action( 'delete_comment', $comment_id );

… and one after deletion:

/**
 * Fires immediately after a comment is deleted from the database.
 *
 * @since 2.9.0
 *
 * @param int $comment_id The comment ID.
 */
do_action( 'deleted_comment', $comment_id );

So you can bind your callback to that:

add_action( 'deleted_comment', function( $comment_id ) {
    delete_comment_meta( $comment_id, 'your_meta_key' );
} );