Remove the `workloads.default_namespace` Helm chart value

[gcapizzi]

The Eirini REST API needed to be told in the request which namespace to deploy workloads to. workloads.default_namespace was used if the namespace was not specified in the request.

None of this is needed in eirini-controller, as it always uses the LRP/Task namespace as its destination namespace. workloads.default_namespace is unused in this codebase and can be safely removed.