There is at least one vdisk mapped to this host, therefore the host object cannot be deleted without the loss of data.
Verify that you are attempting to delete the correct host, if you are sure that this is the correct host, then use lshostvdiskmap to locate the vdisks that are mapped to this host.
If you are sure that these mappings are no longer required, remove them using rmvdiskhostmap, and retry the original rmhost command.