Remove old Windows-only implementation of keeping main outside the rts

Remove old Windows-only implementation of keeping main outside the rts
We now do it for all ways and for all platforms. This was a Windows-only
version that only kept a separate Main.dyn_o for the dynamic linking case.
It had to do that because Windows DLLs are stricter about unresolved symbols
where as for ELF platforms we only run into the problem when we're not using
a Haskell main function.