- secondaryColor control only made sense in fixed-function
- Vulkan provoking vertex is always the first vertex

Bug: b/125909515
Test: dEQP-VK.rasterization.flatshading.*
Change-Id: I9b6826e093076ee7ca5ef6deaf1b1fbd1c2f39e3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27509
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>