Remove `vsa_standalone_settings_page` feature flag

What does this MR do and why?

Removes the feature flag and all related code

References

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MR acceptance checklist

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Screenshots or screen recordings

Feature off (Modal)	Feature on (New page)

How to set up and validate locally

1. Visit Value stream analytics for any group or project (ex. http://gdk.test:3000/flightjs/Flight/-/value_stream_analytics)
2. When creating a new Value stream, the page should redirect to a separate page
3. When editing an existing Value stream, the page should redirect to a separate page

Related to #444848 (closed), #479372 (closed)