Remove Zenodo reference.

[bdice]

Description

Companion PR to https://github.com/glotzerlab/signac/pull/903. We haven't been keeping Zenodo up to date (last updated February 2019) and it gets very few citations (only 4 citations listed on the Zenodo page and 9 on Google Scholar, compared to 95 Google Scholar citations of the 2018 paper). We're going to remove it, to simplify the process of acknowledgement and avoid pointing to outdated content.

Checklist:

• [x] I am familiar with the Contributing Guidelines.
• [x] I agree with the terms of the Contributor Agreement.
• [x] My code follows the code style guideline of this project.

[bdice]

cc: @joaander if this affects citation statistics tracking.

[joaander]

The siganc developer team is free to track citations however they wish.