Removed user identity validation restriction on email form

When a user is added/removed to a subproject, the service require to have him specified by email by applying a check on the form "subject"@"domain"

Entitlement V2 requires a subject to be specified, not an email in case the user is a service principal accounts (or other identities that can be represented in a form of a string).

The check on the email form has been removed to accommodate the new behavior (the service now is checking that the "email" field is present and is in a form of a string)