It was already marked as internal use only, and no longer used
internally. Also, it won't work when we dispatch to PSA.
Remove it before the MD_LIGHT split to avoid a corner case: it's
technically a hashing function, no HMAC or extra metadata, but we still
don't want it in MD_LIGHT really.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>