Remove notes and notes_internal field from work items index

Background

The feature flag to index notes into the work items index was removed in !191369 (merged)

The fields can be cleaned up from the index.

Proposal

The M step was performed in %18.1 in !191376 (merged)

Follow the guide for M+1 to remove a field from advanced search: https://docs.gitlab.com/development/advanced_search/#removing-a-field