Drop yarn from Supervisor devcontainer

Yarn is used in the scripts to update the Supervisor frontend. However,
the frontend project has removed the Supervisor frontend, and we only
ship the frontend in Supervisor for old Core installation.

It is unlikely that we need to update the frontend again in Supervisor.
So simply drop the dependency from the devcontainer. In case we still
need it for one (last) frontend update, we can manually install it.

Author: agners