Remove`issue_assignees_widget` feature flag

What does this MR do and why?

This MR removes issue_assignees_widget FF

Screenshots or screen recordings

No visual changes

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 #328185 (closed)