icp: reorganise SHA2 digest mechanisms

sha2_mech_type_t serves double-duty, as the list of MAC providers and
also the algo type for direct callers to SHA2Init. Until we disentangle
that, reorganise it to make the separation more clear. While we're
there, remove the digest mechs we don't use.

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