Do not delete bdev on update after changing a namespace load balancing group #1851

Job	Run time
build	7m 56s
build-ceph	1m 54s
demo	3m 44s
pytest (cli)	2m 2s
ha (sanity)	4m 6s
pytest (cli_change_lb)	2m 15s
ha (state_transitions)	3m 42s
pytest (state)	2m 9s
ha (state_transitions_both_gws)	4m 24s
pytest (multi_gateway)	2m 34s
discovery (container)	4m 3s
ha (state_transitions_loop)	11m 49s
pytest (server)	2m 37s
discovery (embedded)	3m 54s
ha (state_transitions_rand_loop)	14m 32s
pytest (grpc)	5m 14s
ha (late_registration)	3m 32s
pytest (omap_lock)	4m 38s
ha (late_registration_loop)	14m 13s
pytest (old_omap)	2m 0s
ha (4gws)	5m 44s
pytest (log_files)	3m 39s
ha (4gws_loop)	7m 5s
pytest (nsid)	3m 19s
ha (namespaces)	9m 57s
ha (namespaces_loop)	16m 42s
push-images-to-private-registry	0s
push-images-to-ceph-registry	0s
	2h 27m 44s

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