Remove incorrect localVue usage in 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/callout_spec.js
• spec/frontend/vue_shared/components/expand_button_spec.js
• spec/frontend/vue_shared/components/issue/related_issuable_item_spec.js
• spec/frontend/vue_shared/components/markdown/suggestion_diff_header_spec.js
• spec/frontend/vue_shared/components/notes/system_note_spec.js
• spec/frontend/vue_shared/components/notes/timeline_entry_item_spec.js
• spec/frontend/vue_shared/components/pagination_links_spec.js
• spec/frontend/vue_shared/components/time_ago_tooltip_spec.js
• spec/frontend/vue_shared/directives/track_event_spec.js
• spec/frontend/vue_shared/droplab_dropdown_button_spec.js
• spec/frontend/vue_shared/mixins/gl_feature_flags_mixin_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: #121746 (closed)