EGL: Remove internal enum for NULL driver. This is now updated in Chrome, so we can remove the last uses. Bug: angleproject:2159 Change-Id: Ia31e42b8a685756ee70450be19a52248e3efa92f Reviewed-on: https://chromium-review.googlesource.com/853119 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>