icp: remove digest entry points

For whatever reason, we call digest mechanisms directly, not through the
KCF digest provider. So we can remove those entry points entirely.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16209