Remove `activityBarViews` from commit_sidebar/form.vue

The following discussion from !21067 (merged) should be addressed:

• @mishunov started a discussion: (+1 comment)

  suggestion: I know this has not been introduced by this MR, but can we include a small refactoring while we're at this file? It looks like activityBarViews is not used as an $options anywhere in the view. Can we remove it from the component options?