Rely on jenkinsci organization release drafter configuration (#284)

Release drafter v7 dropped support for `_extends: .github` in the
configuration.  It either needs to be removed or replaced with a more
complete specification like:

`_extends: github:jenkinsci/.github:/.github/release-drafter.yml`

This pull request removes it because the "Enabling CD" instructions at
https://www.jenkins.io/doc/developer/publishing/releasing-cd/#configure-release-drafter
say:

> You can delete any existing Release Drafter configuration ... as that
> will be subsumed by the CD workflow

Testing done

* None.  Has worked well in many repositories, not expecting any issues in this repository

Author: MarkEWaite