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/keyring/api/DummyKeyringModule.java

@@ -109,7 +109,6 @@ public final class DummyKeyringModule {
         "not a real login",
         "not a real password",
         "not a real login",
-        "not a real credential",
-        "not a real key");
+        "not a real credential");
   }
 }