Do not delete bdev on update after changing a namespace load balancing group #1850

Job	Run time
build-ceph	2m 19s
build	8m 45s
demo	3m 44s
ha (sanity)	3m 51s
ha (state_transitions)	3m 33s
pytest (cli)	1m 59s
ha (state_transitions_both_gws)	4m 40s
pytest (cli_change_lb)	2m 12s
discovery (container)	3m 57s
pytest (state)	2m 30s
discovery (embedded)	4m 9s
ha (state_transitions_loop)	11m 36s
pytest (multi_gateway)	2m 37s
ha (state_transitions_rand_loop)	13m 39s
pytest (server)	2m 50s
ha (late_registration)	10m 3s
pytest (grpc)	3m 14s
ha (late_registration_loop)	16m 16s
pytest (omap_lock)	4m 48s
ha (4gws)	6m 7s
pytest (old_omap)	2m 15s
ha (4gws_loop)	6m 54s
pytest (log_files)	3m 29s
ha (namespaces)	9m 34s
pytest (nsid)	4m 8s
ha (namespaces_loop)	16m 48s
push-images-to-private-registry	0s
push-images-to-ceph-registry	0s
	2h 35m 57s

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