Vulkan: Remove provoking-vertex structs from "custom" list This extension has been released. Bug: angleproject:4063 Change-Id: I007c20a895819aa11382837a3ad654839e127365 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3217938 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>