Pull request without forking?

Once your pull requests are accepted, you can safely delete the repo.

Afterwards, simply clone directly from the original upstream repo.

If you have other pull requests to make, then you should make them in their own respective branch, which means you can safely delete the branch you made to manage an accepted pull request (as in "My pull request has been merged, what to do next?")

If you don't have access to create branches on that repository, there is no way to create a pull request without forking.