Remove OMF flags from Makefile

[LightBender]

@thewilsonator @rikkimax

Depends on this PR being merged: dlang/dmd#16422

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8993"

[rikkimax]

It most certainly is meant to be running dmd's testsuite: https://github.com/dlang/phobos/blob/master/.github/workflows/main.yml#L115