Windows: Remove obsolete support for OMF/DMC/Optlink toolchain

[kinke]

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[kinke]

I've noticed that the changes to the create_dmd_release/extras overlay surprisingly aren't applied for the CI artifacts. The problem is that build_all.d clones all repos, incl. the installer repo itself, and uses that tree for the extras (looks like that's the only problematic subtree). For PRs like this from a forked repo, it uses the master branch of installer and its extras. It's only after merging that CI uses the new extras, and might break. FYI @ibuclaw

As a workaround, one can presumably push the branch to the upstream remote directly.