Remove "required" attribute from required multiple checkbox fields

When a webform multiple checkbox field is set to required, a "required" attribute will be added to the markup of each checkbox input element.
The current html spec makes it so that each checkbox has to be checked in order to complete validation (not just one).
See html spec discussion: https://www.w3.org/Bugs/Public/show_bug.cgi?id=9160

Solution:
Ditch html validation by removing the "required" attribute from the input elements. Rely only on webforms own validation.