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.

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

java/google/registry/tools/UpdateKmsKeyringCommand.java

@@ -56,9 +56,6 @@ final class UpdateKmsKeyringCommand implements RemoteApiCommand {
     byte[] input = Files.readAllBytes(inputPath);
 
     switch (keyringKeyName) {
-      case BRAINTREE_PRIVATE_KEY:
-        kmsUpdater.setBraintreePrivateKey(deserializeString(input));
-        break;
       case BRDA_RECEIVER_PUBLIC_KEY:
         kmsUpdater.setBrdaReceiverPublicKey(deserializePublicKey(input));
         break;