Duplicate feature flags

[maddeleine]

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Problem:

We've ended up with two duplicate feature flags, S2N_LIBCRYPTO_SUPPORTS_EVP_KEM AND S2N_LIBCRYPTO_SUPPORTS_MLKEM, essentially, they both test for the existence of MLKEM algorithms. We should figure out how that happened and remove one if necessary.

Need By Date:

N/A

Solution:

Probably remove one of the flags. But first we should understand how we ended up with identical feature flags. I think it was when @alexw91 was removing kyber from out library.

Requirements / Acceptance Criteria:

Out of scope:

[jmayclin]

Historical context: S2N_LIBCRYPTO_SUPPORTS_EVP_KEM was originally added to support Kyber (the pre-standardized variant of MLKEM). We expect that this can now be removed.