Remove useless attachToDocument usage in spec/frontend/issuable

What does this MR do?

As described in &2419 (closed), cleans up our Jest tests from unnecessary attachToDocument usage

Involved specs

• spec/frontend/issuable_suggestions/components/app_spec.js
• spec/frontend/issuable_suggestions/components/item_spec.js
• spec/frontend/issuables_list/components/issuable_spec.js
• spec/frontend/issuables_list/components/issuables_list_app_spec.js

Conformity

• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes: #196757 (closed)