Remove refs from `RegistrationForm`
This aligns the code in `RegistrationForm` with other users of the `Field`
component. (In https://github.com/matrix-org/matrix-react-sdk/pull/2780, I had
thought that this code would be okay to leave alone, but I had missed the usage
of the `Field` value getter.)
Fixes https://github.com/vector-im/riot-web/issues/9172
