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/com/facebook/yoga/YogaConfig.java

@@ -38,8 +38,4 @@ public abstract class YogaConfig {
   public abstract YogaLogger getLogger();
 
   abstract long getNativePointer();
-
-  public abstract void setUseVanillaJNI(boolean useVanillaJNI);
-
-  public abstract boolean useVanillaJNI();
 }