Remove unused P/Invoke interface

Summary:
- Remove YGNodeGetContext and YGNodeSetContext from Native.cs
- https://github.com/facebook/yoga/pull/297#discussion_r93789907

Reviewed By: emilsjolander

Differential Revision: D4365845

fbshipit-source-id: 89b41ec773baa1b4901f637f5e83427c9fba4aed

csharp/Facebook.Yoga/Native.cs

@@ -98,12 +98,6 @@ namespace Facebook.Yoga
 
         #region YG_NODE_PROPERTY
 
-        [DllImport(DllName)]
-        public static extern void YGNodeSetContext(YGNodeHandle node, IntPtr context);
-
-        [DllImport(DllName)]
-        public static extern IntPtr YGNodeGetContext(YGNodeHandle node);
-
         [DllImport(DllName)]
         public static extern void YGNodeSetMeasureFunc(
             YGNodeHandle node,