HPCC-11786 Roxie cored when processing a file notification

Previous fix did not work, as notifier is linked elsewhere. Have to go via
daliHelper to unsubscribe.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6143 from richardkchapman/roxie-superfile-unsubscribe

HPCC-11786 Roxie cored when processing a file notification

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>