Unprepare the temporary index for BackfillEventsTargetTypeForProjects

See 'target_type' parameter for Events API is nil f... (&19098) for context

Currently, project events are created without a set target_type and target_id. This is causing a problem with our Events RESP API as the target_type filter always gives an empty result for project.

Once Finalize `BackfillEventsTargetTypeForProjects` ... (#565789 - closed) has been completed. We can now remove this temporary index.

We have decide not to do the backfill for legacy project events. Therefore, we should unprepare the temporary index meant to support the backfill.

Implementation Guide

1. Unprepare the temporary index as prepared in Schedule temporary index for `BackfillEventsTar... (#565791 - closed)