Remove viewMatrix

Users are supposed to use `view.transform.inverse.matrix`

The view matrix was incorrect anyway, we had forgotten to invert it, but that was okay since we transposed it by accident (due to euclid's use of the row-vector convention), and for 3DOF devices these transforms are mostly rotations so  transposing instead of inverting doesn't lead to visible effects.

components/script/dom/webidls/XRView.webidl

@@ -13,6 +13,5 @@ enum XREye {
 interface XRView {
   readonly attribute XREye eye;
   readonly attribute Float32Array projectionMatrix;
-  readonly attribute Float32Array viewMatrix;
   readonly attribute XRRigidTransform transform;
 };