GitHub now supports leaving the fork network from the repository settings.

So your repository is a fork.

Go to Settings -> General, and then go to the Danger Zone, then click "Leave fork network"

You may now get a confirmation prompt, confirming that you understand this action is permanent and you cannot rejoin the fork network.

Once you understand the implications, you can click on the "I have read and understand these effects" button.

Then you will be prompted to type in your repository name to confirm.

After you do that, you can click the "Leave fork network" button.

Your repository should now should no longer be considered a fork of upstream by GitHub.