Remove calls to MD layer functions from TLS

[mpg]

Currently calls to PSA for hashing in TLS are conditional to MBEDTLS_USE_PSA_CRYPTO. Since we're now auto-enabling that, we don't need the alternative code path any more.

This task is to replace all instances of:

#if defined(MBEDTLS_USE_PSA_CRYPTO)
/* code using PSA */
#else
/* code using mbedtls_md or mbedtls_shaxxx */
#endif

with:

/* code using PSA */

Then removing struct members that are no longer needed because they were only accessed by the removed code.

• tls_prf_generic()
• ssl_calc_verify_tls_sha256()
• ssl_calc_verify_tls_sha384()
• mbedtls_ssl_reset_checksum()
• ssl_update_checksum_start()
• ssl_update_checksum_sha256()
• ssl_update_checksum_sha384()
• ssl_calc_finished_tls_sha256()
• ssl_calc_finished_tls_sha384()
• ssl_handshake_params_init()
• mbedtls_ssl_handshake_free()
• mbedtls_ssl_get_key_exchange_md_tls1_2()
• struct mbedtls_ssl_handshake_params: fin_sha256 and fin_sha512

Depends on: #5156 (to be able to use PSA without #ifdefs).

Note: this task is only about MD functions. PK functions may require more care and will be handled as a separate task. Cipher functions will also covered by other tasks.
Note: this task is only about places where a PSA-based version of the code already exists. Changing other places from MD to PSA is out of scope and will be covered by other tasks.

[mpg]

Closing as this no longer makes sense with the change of strategy (USE_PSA_CRYPTO will remain optional for now).