Validation of RegistrationEmailAsUsername attribute

[spirius]

Background:
I want to have a realm, where username and email attributes have same value, registration is disabled and user is allowed to manage own email address. To achieve this configuration keycloak can be configured as follows

RegistrationAllowed=false
RegistrationEmailAsUsername=true
EditUsernameAllowed=false

As a result, keycloak's account management page will allow user to manage own email address (but not username), and username will be updated automatically by keycloak upon change.

This configuration is of course not very intuitive, because RegistrationEmailAsUsername attribute should make sense only when registration is enabled, but in fact keycloak behaves just as described above. In order to achieve this from GUI, RegistrationAllowed can be temporary enabled / disabled, which will allow setting RegistrationEmailAsUsername to true.

Current implementation of the provider does not allow setting RegistrationEmailAsUsername to true when RegistrationAllowed is false, resulting in validation error: RegistrationEmailAsUsername cannot be true if RegistrationAllowed is false. This change removes that condition.

[mrparkers]

makes sense to me, thanks for the PR!