dotnet20sp1,dotnetsp2,dotnet30: don't override ngen.exe in wine-4.0+
dotnet20sp1,dotnetsp2,dotnet30: don't override ngen.exe in wine-4.0+

This breaks with newer wine (when compiled with mingw).
In any case, it's not needed with wine-4.0.

Tested with `./tests/winetricks-test dotnet` with:
wine-3.0
Wine-4.0
wine-5.0-rc1 (with/without mingw)

Winetricks/winetricks,Winetricks/winetricks