Remove issuable Vue mixins (mixins/issuable.js)

Context: We are currently migrating away from using mixins because 1) mixins hamper readability and 2) composition api is coming in Vue 3 and will be encouraged so we'd like to be ready.

• Relevant section in the GitLab Doc: https://docs.gitlab.com/ee/development/ee_features.html#for-js-code-that-is-ee-only-like-props-computed-properties-methods-etc

• Relevant frontend RFC: gitlab-org/frontend/rfcs#32 (closed)

Problems to solve:

• Replace out mixins/issuable.js with suitable alternatives in the following places:
  • app/assets/javascripts/notes/components/comment_form.vue
  • app/assets/javascripts/notes/components/discussion_locked_widget.vue
  • app/assets/javascripts/notes/components/note_form.vue
• Remove https://gitlab.com/gitlab-org/gitlab/blob/master/app/assets/javascripts/vue_shared/mixins/issuable.js

This issue is a direct follow up on !36773 (comment 386338038)