Remove incorrect localVue usage in spec/frontend/snippets

What does this MR do?

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

Involved specs

• spec/frontend/snippets/components/app_spec.js
• spec/frontend/snippets/components/snippet_header_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: #121744 (closed)