TLS record HMAC 3: remove non-PSA keys

[mpg]

This is the conclusion of a series starting with #5176 and continuing with #5177 and #5178. Now that all computations use PSA, we can remove the legacy contexts from the transform when MBEDTLS_USE_PSA_CRYPTO is defined.

• In struct mbedtls_ssl_transform, remove md_ctx_enc/md_ctx_dec when MBEDTLS_USE_PSA_CRYPTO is defined.
• In mbedtls_ssl_transform_init() and mbedtls_ssl_transform_free(), remove related code when MBEDTLS_USE_PSA_CRYPTO is defined.
• In ssl_tls12_populate_transform(), code using those fields when MBEDTLS_USE_PSA_CRYPTO is defined.

Depends on: #5177 and #5178 - for the legacy context to be no longer needed.

[superna9999]

While #5573 implements this, I think it could go further to a complete removal of md_info in ssl_tls12_populate_transform(), should I create a new issue for that or put a new PR when #5573 is merged ?

[mpg]

Hmm, good question. Strictly speaking, I think that step further would be part of #5210 - remove the remaining dependencies on MD - which we weren't planning on working on right now. OTOH, it's always better to strike the iron while it's hot :) How much work do you think it would be? If it's just one or two smallish commits, perhaps just do it as part of 5573? Otherwise, I'd say open a PR after 5573 is merged. Or you could also just leave a note in 5210 with your thoughts. Up to you!

[superna9999]

It's a small commit, I'll open a PR after #5573 is merged, not need to add more complexity to it.