Moodle: Prevent students showing their private email addresses

A moodle user profile setting allows students to choose to reveal their email address.  In many schools, it may be desirable to remove this option.
The effect of the following hack merely removes these options from the drop down choices in the moodle user profile.

moodle/ user/ editlib.php
Comment out lines 88-89

 //   $choices['1'] = get_string('emaildisplayyes');
 //   $choices['2'] = get_string('emaildisplaycourse');