It's not used anywhere and removing it will make changing traversal
code a bit simpler.

BUG=angleproject:2662
TEST=angle_unittests

Change-Id: I4a430a09ceb538c8b0e5d1bb0a95f3fd7657c276
Reviewed-on: https://chromium-review.googlesource.com/1098671Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>