Remove PR URL variable from Release Drafter config

The variable doesn't seem to be set so we can't use it to make a link to the PR.

Reminders:

[ ] Run make format and make check to make sure the code follows the style guide.
[ ] Add tests for new features or tests that would have caught the bug that you're fixing.
[ ] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
[ ] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
[ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
[ ] If adding new example, please build the docs with make -C doc gallery and push the changes made on doc/gallery. Continuous Integration services build and deploy the docs, but they won't run the examples.
[ ] Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

Reminders for Maintainers:

[ ] Maintainers must run all tests locally before the PR is merged. CIs run only minimal tests and styling checks.

fatiando / rockhound