Remove Katacoda content and config

[AlanGreene]

Follow up to https://github.com/tektoncd/website/issues/366

Once references to the Katacoda scenarios have been removed everywhere we should also remove the following:

• [x] tutorials/katacoda
• [x] katacoda.yaml
• [x] the Katacoda webhook in the website repo settings
• [ ] the Katacoda account/profile (or keep it for future use?)
• [ ] anything else?

Assigning to myself for now /assign /kind cleanup

[AlanGreene]

Katacoda content and config is removed, and in good time too as O'Reilly announced last week that they'll be shutting down katacoda.com on June 15th: https://www.oreilly.com/online-learning/leveraging-katacoda-technology.html