Made mail_sent field (on membership) no longer required, so it can be hi...

In DEV environment an error occurred on the (hidden) field mail_sent when registering for a group.
Because the mail_sent field has the correct default value (0), I removed the 'required' characteristic of the mail_sent field on membership. 
(link: http://localhost:7880/mica/?q=admin/config/group/group-membership/manage/og_membership_type_default/fields)
This was added to feature.