Remove the ability to download service account credentials

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=224034254

javatests/google/registry/testing/FakeKeyringModule.java

@@ -150,17 +150,6 @@ public final class FakeKeyringModule {
         return rdeReceiverKey;
       }
 
-      @Override
-      public String getEncryptedData(String keyName) {
-        throw new RuntimeException(
-            "Fake keyring does not support the retrieval of encrypted data.");
-      }
-
-      @Override
-      public byte[] getDecryptedData(String keyName, String encryptedData) {
-        throw new RuntimeException("Fake keyring does not support decrypting of supplied data.");
-      }
-
       @Override
       public void close() {}
     };