Remove useVanillaJNI flag and usage of fbjni in yoga build

Summary:
## Changelog:

[General] [Yoga] - Use vanilla jni instead of fbjni for all the jni communication

Reviewed By: astreet

Differential Revision: D17808005

fbshipit-source-id: 5e9a1ed73978f519c71c248774a28e5a294e7c7f

java/jni/yogajni.cpp

@@ -4,20 +4,14 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-
-#include "yogajni.h"
 #include "YGJNIVanilla.h"
-#include <fbjni/fbjni.h>
 #include "common.h"
 
-using namespace facebook::jni;
 using namespace facebook::yoga;
 
 jint JNI_OnLoad(JavaVM* vm, void*) {
-  jint ret = YGJNI::registerNativeMethods(vm);
-
   JNIEnv* env;
-  vanillajni::ensureInitialized(&env, vm);
+  jint ret = vanillajni::ensureInitialized(&env, vm);
   YGJNIVanilla::registerNatives(env);
   return ret;
 }