[Platform CI] Get rid of Jenkins X for deployment

Though Jenkins is deployed in the Kubernetes cluster with the official jenkinsci Helm chart, see NXBT-3353, we're still relying on the jenkins-x-platform umbrella chart for the other components, mostly Nexus and ChartMuseum.
As we target complete independence from Jenkins X, we need to use a standard way to deploy these components, such as additional Helm charts.

Moreover, we would like to get completely rid of all Jenkins X related stuff:

• jx calls in the various piprlines, e.g.:
  • jx step git credentials
  • jx preview
  • etc.
• Kubernetes resources installed by either the original "jx install" command run to install Jenkins X or the " jx upgrade platform" command run by the platform-ci pipeline, e.g.:
  • jenkins-x-controllerrole stuff
  • jenkins-x-gcpreviews cronjob
  • All the jenkins.io/v1 resources: environments, apps, pipelineactivities, etc.
  • etc.