Cleanup prominent_create_board_btn feature flag

What does this MR do and why?

This is an extremely long-running experiment that we have never made a decision on. This has been turned on for 50% of users, but there was no clear indication that this was actually beneficial to users. Remove the feature flag and the feature as the experiment was deemed experimentinvalidated - #343415 (comment 1936082349)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

When FF is removed

How to set up and validate locally

• Sign in as root. Visit /Commit451/lab-coat/-/boards/25
• Make sure there is no New board button next to dropdown.

Related to #467417 (closed)