Address vue/no-unused-properties violations in app/assets/javascripts/sidebar/

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What does this MR do and why?

Fixes eslint rule in the following files:

• app/assets/javascripts/sidebar/components/labels/labels_select_vue/dropdown_title.vue
• app/assets/javascripts/sidebar/components/labels/labels_select_widget/dropdown_contents.vue
• app/assets/javascripts/sidebar/components/labels/labels_select_widget/dropdown_contents_labels_view.vue

References

Issue: #525766

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