Remove SPIRV_METAL references from .gn and tests The backend was removed but the references were not. Update ShaderBinaryTest to properly skip tests if shader binaries are unsupported in the current ANGLE backend. Fixed: angleproject:6081 Change-Id: I54bb4080763fbc0dcc2515e71ccd5df5c536db5b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4591046 Reviewed-by: Kimmo Kinnunen <kkinnunen@apple.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>