Remove `cpu_limit`, `memory_limit` and Kubernetes resource patch in track 3

[mmkay]

Enhancement Proposal

As agreed earlier (see #246), once we're in track 3, we want to remove cpu_limit, resource_limit and associated resource patch altogether in favour of Juju constraints setting.

[PietroPasotti]

• Coordinate with Core about Remove k8s compute resource patch cos-coordinated-workers#102 and if necessary pick that up as well as a separate task (they should be independent)
• don't forget to remove/update the docs that reference this feature.