Unprepare the temporary index events target_type for projects

What does this MR do and why?

Remove tmp_idx_events_where_project_target_type_missing as we decided not to backfill.

Closes #566147 (closed)

Reverses the changes made in !203765 (merged)

Migration (up)

➜  rake db:migrate

ci: == [advisory_lock_connection] object_id: 130160, pg_backend_pid: 79313
main: == [advisory_lock_connection] object_id: 130160, pg_backend_pid: 79884
main: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: migrating ==
main: -- index_exists?(:events, :id, {:name=>"tmp_idx_events_where_project_target_type_missing"})
main:    -> 0.0269s
main: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: migrated (0.0364s) 

main: == [advisory_lock_connection] object_id: 130160, pg_backend_pid: 79884
ci: == [advisory_lock_connection] object_id: 130160, pg_backend_pid: 80275
ci: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: migrating ==
ci: -- index_exists?(:events, :id, {:name=>"tmp_idx_events_where_project_target_type_missing"})
ci:    -> 0.0234s
ci: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: migrated (0.0519s) 

ci: == [advisory_lock_connection] object_id: 130160, pg_backend_pid: 80275

Migration (down)

➜  rake db:rollback:main STEP=1

main: == [advisory_lock_connection] object_id: 129960, pg_backend_pid: 17579
main: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: reverting ==
main: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: reverted (0.3915s) 

main: == [advisory_lock_connection] object_id: 129960, pg_backend_pid: 17579

➜  rake db:rollback:ci STEP=1

ci: == [advisory_lock_connection] object_id: 129960, pg_backend_pid: 15370
ci: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: reverting ==
ci: == 20251021141609 RemoveTmpIdxEventsWhereProjectTargetTypeMissing: reverted (0.4332s) 

ci: == [advisory_lock_connection] object_id: 129960, pg_backend_pid: 15370

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