remove backfill file (#6204)

sql/moz-fx-data-shared-prod/telemetry_derived/desktop_retention_aggregates_v2/backfill.yaml

@@ -1,10 +0,0 @@
-2024-09-13:
-  start_date: 2021-01-01
-  end_date: 2024-09-12
-  reason: DENG-4177 - new table, need to backfill back to 2021-01-01 for KPI reporting
-  watchers:
-  - mhirose@mozilla.com
-  - vsabino@mozilla.com
-  - wichan@mozilla.com
-  status: Initiate
-  shredder_mitigation: false