Remove useless attachToDocument usage in spec/frontend/sidebar

What does this MR do?

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

Involved specs

• spec/frontend/sidebar/assignees_spec.js
• spec/frontend/sidebar/components/assignees/assignee_avatar_link_spec.js
• spec/frontend/sidebar/components/assignees/collapsed_assignee_list_spec.js
• spec/frontend/sidebar/components/assignees/uncollapsed_assignee_list_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: #196761 (closed)