Update form/field/usergroup.php

Delete the commented out section in the getRepeatable() method that is confirmed not applicable to this FOFFormFieldUsergroup class. This is a housekeeping commit as the there is no net effect on what the code does. Also effected is form/header/usergroup.php, which will be deleted.
Delete form/header/usergroup.php.

Delete this file as it is not called by /form/file/usergroup.php::getRepeatable(). This is a housekeeping commit as there is no net effect on what the code does.
Merge pull request #288 from bbloom/fixusergroupfield

Fixusergroupfield