Removing every member of a biological repeat is refused with R_USED_ID_DROPPED at used_ids.repeats #283

Found in the review of #282 (merged as dd8e86f); reproduced on 2d1ab30, so it predates that PR.

Steps: save a description; remove every member folder of one biological repeat (e.g. ctrl/rep2 and drug/rep2); answer Yes, remove for each; save.

Actual: the save is refused with R_USED_ID_DROPPED at used_ids.repeats.

Expected (Spec experiment-v1 §10): a removed repeat's ID stays in used_ids (IDs are never reused), and the removal of all its members, once confirmed, publishes. The ID must remain recorded in used_ids.repeats, not be dropped. Add a regression test with the Spec fixtures, and check the records file with the Spec's tools/audit/refrecords.py check.