Project Dataset Delete API

[jachro]

As a renku graph API user, I'd like there's a Project Dataset Delete API.

Acceptance criteria:

• a new DELETE projects/:slug/datasets/:id API to be added to the TG
  • the API to remove relevant parts in the Project graph
  • the API to remove relevant parts in the Datasets graph
• a new DELETE projects/:slug/datasets/:id API to be added to the KG
• the API to call a relevant API on the Core
• the API to call the new API on the TG