Vulkan: Remove unused GetDepthStencilAspectFlagsForCopy. Was incorrect and causing a Clang warning. Bug: chromium:992863 Change-Id: Ieff7b332ee11a6330bd5879d73b53ad374f40664 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1751622 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>