Drop Mono workaround

[nulltoken]

GitErrorSafeHandle.cs is currently implemented that way

// Required on Mono < 3.0.8
// https://bugzilla.xamarin.com/show_bug.cgi?id=11417
// https://github.com/mono/mono/commit/9cdddca7ec283f3b9181f3f69c1acecc0d9cc289
if (handle == IntPtr.Zero)
{
    return null;
}

As we require Mono 3.6+, this is no longer necessary.

It would be worth taking a look at the other SafeHandle types and drop similar workarounds would they have been duplicated.

[carlosmn]

We got rid of this class with #1247, so the workaround is no longer there.