Ensure mount unit pilots when its ZFS key is loaded

Ensure mount unit pilots when its ZFS key is loaded

Drop Before=zfs.mount dependency explicity on generated key-load .service
unit.
Indeed, the associated mount unit is After=<dataset-key-load>.service.
This is thus the mount point which controls at what point it wants to be
mounted (Before=zfs-mount.service in stock generator), but this can be
an automount point, or triggered by another service.
This additional dependency from the key load service is not needed thus.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Didier Roche <didrocks@ubuntu.com>
Signed-off-by: Didier Roche <didrocks@ubuntu.com>
Closes #10477