The enum VertexFormatType is redundant with angle::FormatID.  The Vulkan
back end has already eschewed VertexFormatType, this change updates the
D3D back ends.

BUG=angleproject:2531

Change-Id: I67ea2203ca80be828f4c757a37810fe67a279364
Reviewed-on: https://chromium-review.googlesource.com/c/1263899Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>