Android JNI gen: Don't emit code for empty RegisterNatives()

base/android/jni_generator/testNativeExportsOnlyOption.golden

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file is autogenerated by
 //     base/android/jni_generator/jni_generator.py
 // For
 //     org/chromium/example/jni_generator/SampleForTests
 
 #ifndef org_chromium_example_jni_generator_SampleForTests_JNI
@@ skipping 172 matching lines @@
 
   jstring ret =
 static_cast<jstring>(env->CallStaticObjectMethod(SampleForTests_clazz(env),
           method_id));
   jni_generator::CheckException(env);
   return ScopedJavaLocalRef<jstring>(env, ret);
 }
 
 // Step 3: RegisterNatives.
 
-static bool RegisterNativesImpl(JNIEnv* env) __attribute__((unused));
-static bool RegisterNativesImpl(JNIEnv* env) {
-
-  return true;
-}
-
 #endif  // org_chromium_example_jni_generator_SampleForTests_JNI