Remove FOSS VSA stylesheet

What does this MR do and why?

This MR removes unnecessary CSS from VSA, specifically the FOSS page bundle which didn't provide much value and the pipeline-id class which wasn't in use.

Screenshots or screen recordings

Project level (very minor change)

Before	After

Group level (no changes)

Before	After

How to set up and validate locally

• Browse to VSA and inspect the page
• http://localhost:3000/groups/gitlab-org/-/analytics/value_stream_analytics
• http://localhost:3000/gitlab-org/gitlab-test/-/value_stream_analytics

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #217485 (closed)