It was only used in exactly one instance in VertexArray. Instead we can
cache a bool and avoid needing to pass it around.

Will make signaling dirty easier in the Vulkan back-end.

Bug: angleproject:3539
Change-Id: Ia570aec051a24a5280df49edc4345c54022b46ec
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1663838Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>