Remove issue data from combined index

Once the new index is in place and used for queries, we should be able to free up the space in the old index. A migration can be run to remove all of the issue type documents.

Any documentation describing what each migration does should be updated.

Epic order

1. Remove joins from the Elasticsearch query for project/group scoped issues search
2. De-normalize the issue permission data. <-- will include migration and can be done in multiple MRs
3. Remove joins from the Elasticsearch query for globally scoped issues search
4. Migration to copy issue data to new index
5. Migration to remove issue data from combined index **
6. Rake task changes to support new index **
7. Backend changes for supporting new indexes during Zero Downtime Re-indexing **
8. Admin UI changes to support new index (any time after new index created/data migrated to it)

** must be released in conjunction with the Migration to copy issue data to new index

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