'zfs share -a' should clean noauto exports

This is a follow on to PR #10688 where zfs share -a allows the
sharing of canmount=noauto datasets if they are mounted. However,
when a dataset with canmount=noauto is not mounted, the command
should also purge any existing entries from the exports file.
Otherwise, after a reboot, the nfs server attempts to export the
underlying mountpath, not the dataset. This can lead to a hard hang
for existing client mounts.

Instead of just skipping the adding of an export if not mounted
and canmount=noauto, have it also remove an existing export of the
dataset so that, after a reboot, we don't export an unmounted dataset.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #10747