Remove .issue-box from all issuable lists

.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 ... */.

1. Remove class name from HTML (HAML, Vue.js etc.)

Occurrences:

• !47998 (merged) Static Issue list (dashboard, project, group)
• !47999 (merged) Jira Issue list (project, group) (so-called Issuable List Vue.js app by @kushalpandya)
• !47999 (merged) TestCase list (project, group) (so-called Issuable List Vue.js app by @kushalpandya)
• !48000 (merged) Epic list (group)
• !48001 (merged) Requirements list (project)

2. Remove CSS

• !48002 (merged) app/assets/stylesheets/pages/issuable.scss

Appendix A. Checking

$ git grep issue-box master
master:app/assets/javascripts/issuable_list/components/issuable_item.vue:    <div class="issue-box">
master:app/assets/stylesheets/framework/variables.scss:$issue-box-upcoming-bg: #8f8f8f;
master:app/assets/stylesheets/pages/issuable.scss:    .issue-box {
master:app/views/projects/issues/_issue.html.haml:  .issue-box
master:ee/app/assets/javascripts/requirements/components/requirement_item.vue:    <div class="issue-box">
master:ee/app/views/groups/epics/_epic.html.haml:  .issue-box
$ git show master
commit 3e4c5ee17523eb3885abdd627366a1820e7bd572 (HEAD -> master, origin/master, origin/HEAD)

Appendix B. Why this happened (How this got enabled)

gitlab-foss!9020 (merged) introduced .issue-box and .issue-info-container at %9.0 and then gitlab-foss!!11991 (merged) improved them and enabled this improvements at %9.4. Meanwhile gitlab-foss!21856 (merged) changed the class name.