refactor: remove unused prf hmac impls

[lrstewart]

Release Summary:

Resolved issues:

related to #5143

Description of changes:

It looks like the PRF uses 3 different HMAC implementations, but in reality it only uses one. This PR removes the unused implementations for clarity.

The important code is:

s2n-tls/tls/s2n_prf.c

Lines 392 to 399 in 711ee0d

	const struct s2n_p_hash_hmac *s2n_get_hmac_implementation()
	{
	#if defined(OPENSSL_IS_BORINGSSL) || defined(OPENSSL_IS_AWSLC)
	return s2n_is_in_fips_mode() ? &s2n_evp_hmac_p_hash_hmac : &s2n_internal_p_hash_hmac;
	#else
	return s2n_is_in_fips_mode() ? &s2n_evp_pkey_p_hash_hmac : &s2n_internal_p_hash_hmac;
	#endif
	}

So we appear to have:

libcrypto	hmac impl
awslc-fips	s2n_evp_hmac_p_hash_hmac
awslc	s2n_internal_p_hash_hmac
openssl-1.0.2-fips	s2n_evp_pkey_p_hash_hmac
other	s2n_internal_p_hash_hmac

BUT:

1. FIPS mode is no longer supported for openssl-1.0.2-fips. So s2n_evp_pkey_p_hash_hmac is never used.
2. awslc-fips doesn't actually use an hmac implementation for its prf. We only use the hmac implementation for our "custom" prf, and awslc-fips uses the libcrypto's PRF as required by FIPS 140-3. We branch here, with s2n_libcrypto_supports_tls_prf only/always true for awslc-fips because of this define.

So we ACTUALLY only have s2n_internal_p_hash_hmac.

Call-outs:

What about sslv3?
You might have noticed that we branch on sslv3 before we branch on on whether to use the libcrypto or custom PRF. That's not an issue. The sslv3 prf doesn't use the hmac implementation either (see the source. It's all hashes). Also, nothing we do to sslv3 is going to make it any more FIPS compliant :)

Testing:

Existing tests still pass.

I also proved that the other hmac implementation weren't being used by replacing their "final" methods with errors: c1f7183 Any meaningful use of hmac requires the "final" method, since that's the only way to actually produce output. The CI still passed (see the little green check next to the commit name-- it can be expanded to see the passing tests).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.