Metal: Remove compilation through SPIR-V Direct metal generation is stable. Bug: angleproject:6081 Change-Id: If9e76f61ad38f2fc9963f0181dfd03c99ffa3e2b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4003675 Auto-Submit: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org>