Restore `Field` value getter for `RegistrationForm`
When working on the `Field` validation support, I thought `RegistrationForm`'s
refs would be okay to leave as is, but I missed that they also depended on the
value getter.
For the moment, it's quicker to temporarily revive the value getter to get
registration working.
Fixes https://github.com/vector-im/riot-web/issues/9171
