Remove some ScummVM stuff from the GUI

The first commit removes the mt-32 and midi options from the menu fully.

The second one removes all platforms but windows and ps2. I'm not too sure about this one. Maybe it would be better to just remove the platform selector all together as the user should never need to touch it. It is also made more confusing by the mac version not being detected as a mac version, thus making this even more confusing.