Metal: Remove texture swizzle compile guards Texture swizzle support: - macOS 10.15 (min compile 12.0, min deploy 10.15) - iOS/tvOS 13.0 (min compile 17.0, min deploy 15.0) Bug: angleproject:360147119 Change-Id: If7e59cc094e5bcdb145a54541662d0fe0a989e8d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5803984 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>