Remove jira_association_missing MR widget

What does this MR do and why?

For #465744 (closed)

Remove the "Jira Association Missing" component, which displayed "Merge blocked: a Jira issue key must be mentioned in the title or description.".

References

Please see the triggering issue, which is a follow-up to the original work that left this component in the codebase.

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

N/A, this removes a component.