crypto: drbg - Remove support for HMAC-SHA256 and HMAC-SHA384

crypto: drbg - Remove support for HMAC-SHA256 and HMAC-SHA384

Remove support for the HMAC-SHA256 and HMAC-SHA384 variants of
HMAC_DRBG, leaving only the HMAC-SHA512 variant of HMAC_DRBG.

HMAC-SHA512 is already the default.  The default did used to be
HMAC-SHA256, but several years ago it was upgraded to HMAC-SHA512 "to
support compliance with SP800-90B and SP800-90C".  Given that the point
of crypto/drbg.c is compliance with those standards, and there's also no
technical reason to prefer HMAC-SHA384 in this situation even if
acceptable, there's really no point in offering anything else.

Note: now that only HMAC-SHA512 remains, a lot of unnecessary
abstractions can be removed.  A later commit will do that.  This commit
just straightforwardly removes the HMAC-SHA256 and HMAC-SHA384 code.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>