net/vmnet: Drop ifdef for macOS versions older than 12.0

net/vmnet: Drop ifdef for macOS versions older than 12.0

macOS versions older than 12.0 are no longer supported.

docs/about/build-platforms.rst says:
> Support for the previous major version will be dropped 2 years after
> the new major version is released or when the vendor itself drops
> support, whichever comes first.

macOS 12.0 was released 2021:
https://www.apple.com/newsroom/2021/10/macos-monterey-is-now-available/

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240629-macos-v1-4-6e70a6b700a0@daynix.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>