Delete all Braintree code

We never launched this, don't planning on launching it now anyway, and it's rotted over the past two years anyway.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202993577

java/google/registry/keyring/kms/KmsKeyring.java

@@ -64,7 +64,6 @@ public class KmsKeyring implements Keyring {
   }
 
   enum StringKeyLabel {
-    BRAINTREE_PRIVATE_KEY_STRING,
     ICANN_REPORTING_PASSWORD_STRING,
     JSON_CREDENTIAL_STRING,
     MARKSDB_DNL_LOGIN_STRING,
@@ -150,11 +149,6 @@ public class KmsKeyring implements Keyring {
     return getString(StringKeyLabel.JSON_CREDENTIAL_STRING);
   }
 
-  @Override
-  public String getBraintreePrivateKey() {
-    return getString(StringKeyLabel.BRAINTREE_PRIVATE_KEY_STRING);
-  }
-
   /** No persistent resources are maintained for this Keyring implementation. */
   @Override
   public void close() {}