Remove legacy custom value stream components and docs

Proposal

Once we have rolled out the VSA path navigation, we should remove the legacy custom value stream form.

TODO

• Remove old stage table components
• Remove old custom vsa components
  • Remove ee/app/assets/javascripts/analytics/cycle_analytics/components/custom_stage_form.vue.
  • Remove ee/app/assets/javascripts/analytics/cycle_analytics/components/add_stage_button.vue
  • Remove the ee/app/assets/javascripts/analytics/cycle_analytics/store/modules/custom_stages module
  • Remove related specs
• Update VSA stage docs
  • Docs: remove content about the old custom stage form, such as https://docs.gitlab.com/ee/user/group/value_stream_analytics/#adding-a-stage
  • Docs: update any missing information on hiding, reordering and adding label based stages with the new UI