Remove search_work_items_index_notes flag

What does this MR do and why?

This MR is removing the search_work_items_index_notes flag and code. The flag is not enabled and the code will not be kept. An alternative approach was chosen

References

• [Feature flag] Rollout of `search_work_items_in... (#510136 - closed)
• Advanced issue search should allow searching in... (#513146 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. setup gdk for Elasticsearch
2. index everything, make sure you have some issues in the database
3. make sure the index does not contain any documents with notes field

Note: if you already have ES setup and data you might see notes data in the index

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.