Removes CustomEvent polyfill and tests

What does this MR do?

Removes unused CustomEvent polyfill. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7204#note_20780699

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

//cc @alfredo1