Remove unused libcrypto HMAC code from s2n_prf

[goatgoose]

Description of changes:

The custom PRF implementation in s2n-tls is designed to swap the underlying HMAC implementation based on the FIPS mode. Normally, the custom HMAC implementation in s2n-tls is used, but if s2n-tls is operating in FIPS mode, the implementation is swapped to the libcrypto HMAC implementation.

However, after #4020, the PRF implementation itself is swapped to the libcrypto PRF implementation when s2n-tls is operating in FIPS mode. As such, the libcrypto HMAC implementation in s2n_prf is no longer needed, so it's removed in this PR.

Call-outs:

• This change does impact the HMAC implementation for OpenSSL-FIPS in the PRF. Currently, OpenSSL-FIPS uses our custom PRF implementation backed with the HMAC implementation from the libcrypto. In all other usages of HMAC, OpenSSL-FIPS uses our custom HMAC implementation. Now, OpenSSL-FIPS also uses our custom HMAC implementation in the PRF.
• hmac->cleanup and s2n_prf_wipe() currently call s2n_hmac_reset(). Resetting doesn't actually clean/wipe the state, though. Instead, it prepares the state for a future HMAC calculation with the same key and algorithm. It seemed more correct to actually wipe the state in these places instead, so I changed it.

Testing:

Existing s2n_prf unit tests.

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

[goatgoose]

Postponing this change until after the libcrypto HMAC implementation is added to s2n_hmac.