clean up unused webhooks

[orbeckst]

We have a number of outdated webhooks that should probably be removed from https://github.com/MDAnalysis/mdanalysis/settings/hooks

• travis-ci.org (create, delete, issue_com...)
• mdanalysis.kolster.onlin ? (gollum, issues, and pull requests)

I am pretty sure they can be deleted but please have a look 👀