[12355] Remove discourse

[isidzukuri]

What? Why?

• Closes #12355

eight years old. I think it was used with the Fair Food Forum which is now not used anymore. So we can take it out.

What should we test?

• not logged in user should be able to view public pages of website
• user should be able to login
• logged in user should be able to view all pages of website

Changelog Category (reviewers may add a label for the release notes):

• [x] User facing changes
• [x] API changes (V0, V1, DFC or Webhook)
• [ ] Technical changes only
• [ ] Feature toggled