Metal: Remove shared event related ifdefs Shared events are supported by iOS 12, macOS 10.14, included in the minimum versions iOS 15, macOS 10.15. Bug: angleproject:360147118 Change-Id: I5b62dade045f9d50864ef1906cb22a18fd09e4da Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5797150 Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Reviewed-by: Alexey Knyazev <lexa.knyazev@gmail.com> Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>