whitout any kind of warranty..... otherwise ignore my post.
in order to delete any registered chassis, as fair as i remember, you have first to romove it in VCS.
example form a command reference manuals:
vcs logical-chassis enable default-config
Use the no form of this command to remove a node from an existing cluster, remaining in logical chassis cluster mode.
no vcs logical-chassis enablerbridge-id rbridge-id default-config