Remove incorrect localVue usage in ee/spec/frontend/vue_shared

What does this MR do?

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

Involved specs

• spec/frontend/vue_shared/components/gl_modal_vuex_spec.js
• spec/frontend/vue_shared/components/markdown/field_spec.js
• spec/frontend/vue_shared/components/markdown/suggestion_diff_row_spec.js
• spec/frontend/vue_shared/components/user_avatar/user_avatar_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: #195869 (closed)