With Mono, Environment.OSVersion returns the macOS version when running in the
simulator. On Catalina, Mono returns 19.* as the Environment.OSVersion, which
means that the check for major version = 15 in the simulator turns out to be a
constant value now (since we don't support running in the simulator on a
4-year-old macOS version).
With .NET, Environment.OSVersion returns the iOS version instead, which means
that the check would have to be reworked.
However, since these version checks have effectively been constant for a few
years now, we can just remove the checks and the resulting unreachable code.
2b96b9ac5f