driver-only ECC: TLS: avoid use of mbedtls_ecp_point_write_binary() (with USE_PSA)

[mpg]

Context: see #6839. This is a step towards sub-goal ECP.TLS.
Depends on: #7073

Currently TLS uses mbedtls_ecp_point_write_binary() in one place: client-side in the 1.2 static ECDH key exchanges, where we have a public key as a pk_context (from the peer's certificate) and want it in the PSA public key export/import format.

After #7073, we'll already have a copy of the key in this format inside the pk_context, so a simple memcpy() will do the trick. We can also get other information (ecdh_bits and ecdh_psa_type fields of the handshake structure) directly from the pk_context.

This task is to remove the call to mbedtls_ecp_point_write_binary() and all other ECP functions from ssl_get_ecdh_params_from_cert() when MBEDTLS_USE_PSA_CRYPTO is enabled.

[valeriosetti]

@mpg I was checking the code, but it seems to me that what is being requested by this issue is already addressed by some previous PR (#7554):

mbedtls/library/ssl_tls12_client.c

Line 2037 in 2e7d572

memcpy(ssl->handshake->ecdh_psa_peerkey, peer_pk->pub_raw, peer_pk->pub_raw_len);

Am I correct?

[mpg]

As for #7406, I agree the main part has been done. Unlike #7406, I'm not seeing opportunities for further simplifications here, so I'm closing this issue.