Delete unused default registrar referral URL

We now require the URL to be filled out by all registrars and so there is no
default needed.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214774054

java/google/registry/config/RegistryConfigSettings.java

@@ -81,7 +81,6 @@ public class RegistryConfigSettings {
     public String greetingServerId;
     public List<String> registrarChangesNotificationEmailAddresses;
     public String defaultRegistrarWhoisServer;
-    public String defaultRegistrarReferralUrl;
     public String tmchCaMode;
     public String tmchCrlUrl;
     public String tmchMarksDbUrl;