Fix cell removal by inserting a more final state than AFS_CELL_FAILED that
indicates that the cell has been unpublished in case the manager is already
requeued and will go through again. The new AFS_CELL_REMOVED state will
just immediately leave the manager function.
Going through a second time in the AFS_CELL_FAILED state will cause it to
try to remove the cell again, potentially leading to the proc list being
removed.