Front-end detection still in place and will be removed in a follow-up.
Removes the Vulkan feature and the special clear handling.

Bug: angleproject:4959
Change-Id: I5d44c3f7dbdb49d8aa0375f54b7148df09732ba2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2363208Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>