delete only waiting jobs
Related to https://github.com/huggingface/datasets-server/issues/2387
As per [internal conversation](https://huggingface.slack.com/archives/C04L6P8KNQ5/p1707031392098359), only waiting jobs should be deleted on backfill to avoid started processes without a related Job record.
 
delete only waiting jobs: Related to https://github.com/huggingface/datasets-server/issues/2387
As per [internal conversation](https://huggingface.slack.com/archives/C04L6P8KNQ5/p1707031392098359), only waiting jobs should be deleted on backfill to avoid started processes without a related Job record.
 