Clean unnecessary 'attachToDocument' in 'spec/frontend/diffs'

Summary

As described in &2419 (closed), we need to clean up our Jest tests from unnecessary attachToDocument usage

Risks

No risks

Involved specs

• spec/frontend/diffs/components/compare_versions_spec.js
• spec/frontend/diffs/components/diff_file_header_spec.js
• spec/frontend/diffs/components/diff_gutter_avatars_spec.js
• spec/frontend/diffs/components/edit_button_spec.js