The register assignment stuff only applies to the D3D back-end.
Cleans up the GL back-ends use of PackedVarying, and will lead
to future cleanups relating to packing varyings.

BUG=angleproject:1123

Change-Id: Iaaa5fc03577e5b61ea6ae76ee1e15ad608037f34
Reviewed-on: https://chromium-review.googlesource.com/295190Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>