Remove unused VSA stage code

What does this MR do and why?

This MR is long overdue. Around GitLab %15.0 the group-level VSA feature frontend was significantly re-architected and the backend code was not cleaned up properly. This MR removes unused services, controller actions and model-level features. The removals are not user-facing, these features might have been used before %15.0.

We removed the following items:

• From the Stage model, remove the RelativePositioning module because we don't use it anymore. We set the position for each stage at once here: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/analytics/cycle_analytics/value_streams/create_service.rb#L72
• Remove the old /value_stream_analytics/stages/... route. We always go via a value stream: /value_stream_analytics/value_streams/1/stages
• Remove the individual Stage services, we update stages via the ValueStream services using accepts_nested_attributes.
• Fix the FE test fixture generation after the route change (it was outdated).

I tested this change on both group-level and project-level VSA pages.

1. Go to project > analytics > value stream.
2. The page should load. (probably will show 0 values)

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1. Ensure you have an ultimate license.
2. Go to group > analytics > value stream.
3. The page should load.
4. Create a new value stream.

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