Remove .issue-box from Issuable list Vue.js App

What does this MR do?

Removes %div.issue-box from Issuable List Vue.js App. This change affects Jira integrated Issue List ("Enterprise Edition") and TestCases List ("Enterprise Edition"), while the change is under /app.

Quoting #284602 (closed):

.issue-box can be removed since .issue-box is just for display: flex, duplicating of issuable-info-container, which has flex: 1; display: flex; /* and more ... */.

Screenshots (strongly suggested)

There should be no (visual) regression:

Jira integrated Issue List

Test Cases

TBD

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Part of #284602 (closed)