Remove epics from label priority sort

What does this MR do?

Update docs to remove epics as they don't use label priority as a sorting mechanism.

#205149 (comment 468624170)

Related issues

Closes

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#move-or-rename-a-page

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• Update the link in features.yml (if applicable)
• Assign one of the technical writers for review.