Remove `unchanged_label` from sample

Remove unchanged_label from sample as it it not used, see https://gitlab.com/teklia/atr/dan/-/blob/9bb952e9a649cd28909910f41c5dcfa167df4d9d/dan/manager/dataset.py

changed milestone to %DAN-P2: Clean code and remove unused code

added P2 Quick Win labels

assigned to @melodie.boillet

mentioned in merge request !144 (merged)

closed