Redirect Registrar.referralUrl UI actions to url field

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196597051

java/google/registry/config/files/default-config.yaml

@@ -58,9 +58,6 @@ registryPolicy:
   # Default WHOIS server used when not specified on a registrar.
   defaultRegistrarWhoisServer: whois.domain-registry.example
 
-  # Default referral URL used when not changed by a registrar.
-  defaultRegistrarReferralUrl: https://www.domain-registry.example
-
   # Mode TMCH should run in (PRODUCTION for production environments, PILOT for
   # all others including sandbox).
   tmchCaMode: PILOT

java/google/registry/config/files/nomulus-config-production-sample.yaml

@@ -24,7 +24,6 @@ registryPolicy:
     - placeholder
     - placeholder
   defaultRegistrarWhoisServer: placeholder
-  defaultRegistrarReferralUrl: placeholder
   tmchCaMode: PRODUCTION
   tmchCrlUrl: http://crl.icann.org/tmch.crl
   tmchMarksDbUrl: https://ry.marksdb.org

java/google/registry/config/files/nomulus-config-unittest.yaml

@@ -6,7 +6,6 @@ registryPolicy:
     - notification@test.example
     - notification2@test.example
   defaultRegistrarWhoisServer: whois.nic.fakewhois.example
-  defaultRegistrarReferralUrl: http://www.referral.example/path
 
 datastore:
   commitLogBucketsNum: 3