Remove notes and notes_internal from work_items index mapping

What does this MR do and why?

search_work_items_index_notes and advanced_search_work_item_uses_note_fields flags and all related code to index notes and notes_internal was removed. This MR removes the fields from the index.

Following this guide, this is MR 1: https://docs.gitlab.com/development/advanced_search/#removing-a-field

The next MR will be in %18.2, tracked in Remove notes and notes_internal field from work... (#542800 - closed)

References

Related to #510136 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. enable elasticsearch in gdk
2. index the instance from scratch: bundle exec rake gitlab:elastic:index
3. ensure that the work items index mapping does not contain the two removed fields: notes and notes_internal

curl localhost:9200/gitlab-development-work_items/_mappings | jq

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.