Bug 1219027. Fix unbinding transformfeedback when deleting it. r=djg

I believe this was the original intention of the code.
This fixes deletion conformance tests.

dom/canvas/WebGL2ContextTransformFeedback.cpp

@@ -41,7 +41,7 @@ WebGL2Context::DeleteTransformFeedback(WebGLTransformFeedback* tf)
         return;
 
     if (mBoundTransformFeedback == tf)
-        BindTransformFeedback(LOCAL_GL_TRANSFORM_FEEDBACK, tf);
+        BindTransformFeedback(LOCAL_GL_TRANSFORM_FEEDBACK, nullptr);
 
     tf->RequestDelete();
 }