Your best bet is to do this with a user plugin that can both remove the unnecessary field from the form and fill the email in automatically with something autogenerated to get past the requirement.

http://joomla.stackexchange.com/questions/7392/joomla-registration-remove-unneeded-fields should help you with the basics of creating a user plugin and removing the email fields.

Likewise, if you use my answer to that question, you can add in an automated email address in `onUserBeforeSave` which should get the record past the model's validation and store the user "without" an email.