[Platform CI] Clean up Jenkins X related stuff

Though NXBT-3614 has removed the call to "jx upgrade platform" when deploying platform-ci, we still have some Jenkins X related stuff that we want to get rid of:

• The various "jx" calls in the Jenkins pipelines, e.g.:
  • jx step git credentials, can be easily reimplemented
  • jx preview, will probbaly need NXBT-3394
  • 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.:
  • the jx namespace
  • expose/cleanup stuff
  • jenkins-x-controllerrole stuff
  • jenkins-x-gcactivities stuff
  • jenkins-x-gcpods
  • jenkins-x-gcpreviews cronjob
  • Jenkins X secrets such as jx-pipeline-git-github-git
  • All the jenkins.io/v1 resources: environments, apps, pipelineactivities, etc.
  • etc.
• ghcr.io/jenkins-x/builder-nodejs image in the nodejs pod template
• Rename the jx-platform-builders repository?
• Rename the nuxeo-platform-jx-bot GitHub user?