add a `jx gitops delete jenkins` command

removing a jenkins server by hand is a little involved right now. you need to remove:
* the `helmfile/$jenkins` directory and files;
* the reference to `helmfile/$jenkins/helmfile.yaml` from `helmfile.yaml`
* the reference to $jenkins and all the imported projects from `.jx/gitops/source-config.yaml`

it would be nice to have a simple CLI command to do all that for you - the converse of `jx gitops add jenkins` basically