Update Epic issue item styling to match with related issues styling

What does this MR do?

This MR does following changes to Issues List present under Epics;

• Flattens the design to match with that of Related Issues list design within Issues.
• [ ] Ensure that remove button is shown only on hover on non-touch clients (moved to https://gitlab.com/gitlab-org/gitlab-ee/issues/7842).

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added/updated for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7600 (closed)