Remove incorrect localVue usage in spec/frontend/diffs

What does this MR do?

As described in &2360 (closed), cleans up our Jest tests from unnecessary localVue usage

Involved specs

• spec/frontend/diffs/components/diff_gutter_avatars_spec.js
• spec/frontend/diffs/components/edit_button_spec.js
• spec/frontend/diffs/components/hidden_files_warning_spec.js
• spec/frontend/diffs/components/no_changes_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: #121741 (closed)