removeShard returns the number of remaining collections to be migrated. 
Due to this condition the viewless timeseries collection are ignored. In fact for legacy timeseries collections the view namespace was reported with type "timeseries" while the system.bucket was reported as "normal" type. Instead, for viewless timeseries we only have on namespace, and it is always reported type "timeseries" in the output of listCollections/listClusterCatalog.