Update interfaces/webxr.idl (#17374)

Source: https://github.com/tidoust/reffy-reports/blob/2bd98c3/whatwg/idl/webxr.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/547080935
diff --git a/interfaces/webxr.idl b/interfaces/webxr.idl
index f9bfdce..4af1c96 100644
--- a/interfaces/webxr.idl
+++ b/interfaces/webxr.idl
@@ -203,8 +203,6 @@
              optional XRWebGLLayerInit layerInit)]
 interface XRWebGLLayer {
   // Attributes
-  [SameObject] readonly attribute XRWebGLRenderingContext context;
-
   readonly attribute boolean antialias;
   readonly attribute boolean ignoreDepthValues;