Remove unused .issue-box CSS

What does this MR do?

Removes unused .issue-box CSS.

$ git grep issue-box
app/assets/stylesheets/framework/variables.scss:$issue-box-upcoming-bg: #8f8f8f;
app/assets/stylesheets/pages/issuable.scss:    .issue-box {
changelogs/unreleased/284602-remove-issue-box-static.yml:title: Remove .issue-box from static (classic) Issuable list
ee/changelogs/unreleased/284602-remove-issue-box-epic.yml:title: Remove .issue-box element from Epics (list)
ee/changelogs/unreleased/284602-remove-issue-box-requirements.yml:title: Remove .issue-box element from Requirements (list)
ee/changelogs/unreleased/284602-remove-issue-box-vue.yml:title: Remove .issue-box from Issuable list Vue.js App
$ git show | head -1
commit d18dad07dc8b520635ed0af570c68857d33bd283

Does this MR meet the acceptance criteria?

Conformity

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

Very final part of #284602 (closed)