Remove value_stream_dashboard_on_off_setting FF

What does this MR do and why?

This MR removes the value_stream_dashboard_on_off_setting feature flag. The flag is default enabled since %16.4.

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 #411223 (closed)