The easy solution is to comment out that line by adding 2 slashes in front of it ("//" - without the quotes).
The reason for that line of code is to prevent conflicts from multiple copies/versions of mootools running at the same time by making sure only one copy/version is active.
Option 1:
// wp_dequeue_script( 'mootools' );
Option 2:
/* wp_dequeue_script( 'mootools' ); */
Both will comment out that function call and prevent it from running.