If you wrote most of the code yourself and received only minor contributions (which is common for most OS projects) it is possible to remove those contributions

If you wrote most of the code yourself and received only minor contributions (which is common for most OS projects) it is possible to remove those contributions and rewrite them, legally it would be hard to challenge. We almost had to do this once for our own proprietary codebase as we discovered during due diligence that we didn't have a proper copyright transfer agreement from several of our interns. Fortunately we were able to resolve this with them (by paying them some extra money to sign a CLA), if it wouldn't have worked we would've removed and rewritten the code instead. It's silly but legally it's doable. Of course this strategy doesn't work if a large part of your codebase originates from third-party contributors, but from my experience Github projects receive mostly small/cosmetic PRs from non-core developers (not saying that those contributions are not valuable btw).