Followup MR to Refactor issues controller

What does this MR do and why?

This is a followup MR to refactor issues controller !107492 (comment 1218480234)

Uses a pre-existing method and removes the newly added method

Issue link - #386574 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Open rails console by running bin/rails console within GDK directory.
2. Enable the feature flag :hierarchy_db_restrictions on a project; Feature.enable(:hierarchy_db_restrictions).
3. Enable the feature flag :okrs_mvc on the same project; Feature.enable(:okrs_mvc).
4. Visit issues list page for the project by navigating to http://gdk.test:3000/gitlab-org/gitlab-test/-/issues.
5. Create an objective by clicking on dropdown button next to New issue and selecting New objective from the dropdown.
6. Once objective is created, click on it to visit its Work Item page, you should be able to see Tree View below the description.
7. Use Add dropdown button create/add key results.
8. Go to Issues list and click newly created key result and you should be navigated to work-item show page.
9. Assign a KR to yourself and go issues dashboard screen by clicking issues icon from the top navbar
10. On click of KR, you should be navigated to work item show page

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.