Cease attempt to unexport ZFS when moving backup to a different zpool

1. The export is done via set sharenfs which means we shouldn't need
   to manually manage exports

2. This part of the script does not work because it tries to unexport
   the old export but by looking up the db record that we have already
   updated to refer to the new zpool.

ROOT/DEBIAN/control

@@ -3,7 +3,7 @@ Priority: optional
 Section: otherosfs
 Maintainer: Chemistry COs <support@ch.cam.ac.uk>
 Architecture: all
-Version: 0.9-ch108
+Version: 0.9-ch109
 Depends: zfs-dkms, postgresql-13 | postgresql-9.5 | postgresql-9.4 , liblockfile-simple-perl, libdbi-perl, libjson-perl, libzfs-perl-chem, libnet-openssh-perl, libdbd-pg-perl, mbuffer, rsync, nfs-kernel-server, pv, libwww-curl-perl, libformat-human-bytes-perl
 Recommends: backup-scheduler-rsync
 Description: a backup system using ZFS (repository 'backup-scheduler')

ROOT/usr/lib/chem-zfs-backup-server/move-machine-to-zpool.sh

@@ -71,12 +71,5 @@ echo Re-enabling backup task
 dosql "update host set disabled='f' where hostname='$FQDN';"
 # Tidy up
 echo Removing old ZFS, snapshots etc
-# need to try to unexport nfs here, but failure isn't a worry
-set +e
-EXPORTS=`dosql "select zfs_target from backup_task where backup_task_id in (select backup_task_id from backup_task natural join host where hostname='$FQDN')"`
-for EXPORT in $EXPORTS ; do
-  exportfs -u $FQDN:/$EXPORT
-done
-set -e
 for SN in `zfs list -r -t snapshot -H -o name $SOURCE/$FQDN ` ; do for H in `zfs holds -H $SN | awk ' { print $2 ; } '` ; do zfs release $H $SN ; done ; done
 zfs destroy -r $SOURCE/$FQDN