Even if you forked on GitHub, you can ask the staff to "detach" or "extract" your fork from the existing network. Detaching only un-forks your repo, whereas extracting makes your fork the new root and preserves any sub-forks thereof. They will happily do it upon request.

Ooh, thanks for posting this. I have a project that I forked so I could use the base code from it, but it really doesn't make sense as a fork and confuses people. (It's a deployment plugin, and my fork just installs a different piece of software than the original)