If it's a copyright violation (be sure that it ACTUALLY is!) they will remove content in response to a DMCA request, but any forks will only be removed if you manually find them and issue a request for each fork. This isn't very useful if you accidentally uploaded your own copyrighted material though, since that's not a violation you could issue a notice for.
You have to be the copyright holder or their representative, so no, it would technically be illegal to DMCA yourself for violating someone else's IP. If you asked github support nicely they might help, though.
