remove obsolete code

bubble-server/src/main/resources/message_templates/en_US/server/pre_auth/ResourceMessages.properties

@@ -165,7 +165,6 @@ err.user.noAdmin=No admin account exists, cannot create another account
 err.user.noSoleNode=Cannot create account, self-node was never initialized and multiple nodes exist
 err.user.setSelfNodeFailed=Cannot create account, initialization of self-node failed
 err.uuid.invalid=UUID is invalid
-err.referralCode.invalid=Referral code is not valid
 err.registration.disabled=Account registration is not enabled on this Bubble
 err.register.alreadyLoggedIn=Cannot register a new account when logged in
 err.name.registered=Username is already registered