Remove leftover merge_request documents from the main index

Background

We have already moved the merge_request to a separate index and deleted the merge_request documents from the main index. But looks like there are still 27157 merge_request documents left in the main index.

Proposal

Just to make sure documents are moved from the main index to the separate index, first reindex all the merge_requests from the main index. Then write a migration to remove these merge_request documents from the main index.