Remove tracking of required fields

The field required&filled status is now encapsulated in the valid property due to /r/2351.
This means that UIs no longer need to check that all required fields are filled.
So the page no longer needs to track required fields either.

Testing Done:
Compiled and ran `ninja test`. Also opened Request Fields from Demo protocol, removed the required string or entered an invalid email, and confirmed the OK button disabled correctly.

Reviewed at https://reviews.imfreedom.org/r/2352/