waitlist doesn't suppress fee and payment fields

if an event reaches waitlist status and people are invited to register for the waitlist, the payment fields should be suppressed – but are not.

this is similar to CRM-17304 – but that is filed against 4.6.x and suggests that it's working on the first registrant but not additional registrants. in 4.7.6 I'm seeing the problem on the first registrant as well.