Disable plantuml from being managed by helm in `gprd` and remove it entirely

Part of gitlab-com/gl-infra/delivery#1962 (closed)

With MR tanka-deployments!201 (merged) we want to move plantuml from being managed by helm in this repository, to being managed by tanka and captured in jsonnet instead. In order to do this, we add it to tanka-deployments, and remove it from this repo.