[RFC] Amiga/VMS cleanup

The removal of most items was pretty straightforward, but I have one question. I noticed that `runtime/doc/develop.txt` states this:
```
Function names can not be more than 31 characters long (because of VMS).
```
Does anyone know if this is still true? If it's not, I'd like to remove it.
Not sure if you're saving this for another PR, but it's redundant to mention MS-DOS. Any "DOS" support we have is an accident of the fact that we (will) support Windows.