Replace kebab menu with x for work item child

What does this MR do and why?

This MR replaces the kebab menu with the x button for the Child Objectives and Key results and Linked Items widgets. The designs can be found here.

Note: This MR only replaces the close button. Rest of the metadata will be moved using #428901 (closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

Prerequisites:

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable Feature.enable(:linked_work_items)

Steps to verify:

1. Login with any user
2. Go to any Project > Issues > List
3. Create an new Objective under New issue split button
4. Go to Objective detail page
5. Under Child Objectives and Key results or Linked Items, click on Add button to open the form
6. Fill the details and add few items
7. Click on the close button after hovering on any item
8. Check if the item is removed

In addition to above flow, it also affects the Key results and Tasks feature.

For Tasks:

1. Add a Task as mention Add a Task docs under an Issue
2. Open the Task
3. Click on the close button after hovering on any item
4. Check if the item is removed

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 #427603 (closed)