Remove Excalidraw backend

[hero101]

Description

As a develop I want to have only one version of the whiteboards backend So that I need to support only one implementation, without worrying about breaking changes

Acceptance criteria

• [ ] Whiteboard backend implementation is removed
• [ ] All dependencies are removed
• [ ] socket.io package is removed
• [ ] Variables under collaboration.whiteboards are removed
• [ ] env variable related to this implementation are removed from the infrastructure repo
• [ ] Any existing routing manifests in Oathkeeper and Traefik are removed

Additional Context

@hero101

I do not believe this risks any regressions, since the implementation has not been used for over 6 months. Furthermore, the whole functionality is now migrated to a separate microservice, so maintaining this implementation makes no sense.

Areas that will be affected

None

[hero101]

@SimoneZaza I suggest this to be done the next Sprint, since it's no risk, has not been used for 6 months, no testing is required, low effort