Commit 985c33b1

 added DMU_BACKUP_FEATURE_BLAKE3 but it is not used by
the code.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Issue #13795
Closes #13796