Index removal: index_ci_job_variables_on_partition_id_job_id

Summary

The index index_ci_job_variables_on_partition_id_job_id was originally introduced to support our CI partitioning efforts related to FKs (#498968 (comment 2454944661)). It is no longer needed.

Proposal

Double check that the index can be removed and confirm that there is sufficient coverage from other indexes. Since the history of this index is known and it's likely redundant, we don't need to do extensive index usage/query investigation.

Drop the index and recover 12GB of disk storage.