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/api/KeyModule.java

@@ -120,10 +120,4 @@ public final class KeyModule {
   static String provideJsonCredential(Keyring keyring) {
     return keyring.getJsonCredential();
   }
-
-  @Provides
-  @Key("braintreePrivateKey")
-  static String provideBraintreePrivateKey(Keyring keyring) {
-    return keyring.getBraintreePrivateKey();
-  }
 }