Remove TODO comment from sskdf.c
Remove TODO comment from sskdf.c

Fixes #12993

The implementation follows the standards/recommendations specified by https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Cr2.pdf.

Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12999)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl