Delete Open Policy Agent (OPA) integration

[Neumann-Nils]

Description

As karydia does not make use of OPA anymore, the integration should be deleted. This includes all snippets in code, documentation and auto-generated dependencies.

Resolves #134. Closes #35.

Checklist

Before submitting this PR, please make sure:

• [x] your code builds clean with make
• [x] your code lets succeed unit tests with make test
• [x] your code lets succeed integration tests
• [x] you have documented new or changed features