Stop writes to Epics Index

Background

Since we are using work_items index for epics search, we need to remove the index for that we are stopping the writes to index first.

Proposal

• Remove the epic document indexing logic in ee/app/models/concerns/search/elastic/epics_search.rb
• Remove the logic to index synced_epics from ee/app/models/concerns/search/elastic/issues_search.rb