Remove index_deployments_on_project_id_and_ref

Problem

See Reduce main database load by optimizing `deploy... (&10169)

As of 2023-09-13, this has a usage rate of 0 over the last 60 days according to Thanos.

Proposal

https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#drop-indexes-asynchronously