This removes VertexArrayVk::updateIndexTranslation. Turns out this
helper function wasn't that helpful.

Bug: angleproject:3539
Change-Id: Ia4573219073261767e9d215ed4227233c23cbfaa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1660639Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>