Remove calls to MD layer functions from X.509

[mpg]

Currently calls to PSA for hashing in X.509 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 two instances of:

#if defined(MBEDTLS_USE_PSA_CRYPTO)
/* code using psa_hash */
#else
/* code using mbedtls_md */
#endif

with:

/* code using psa_hash */

While at it, the PSA version, which currently uses the multi-part API (with psa_hash_operation_t) can be simplified to use the single-part API psa_hash_compute().

• replaced in x509_crt_check_signature()
• replaced in mbedtls_x509write_csr_der_internal()

Depends on: #5156 (to be able to use PSA without #ifdefs).
See also: #5157

Note: this task is only about MD functions. PK functions may require more care and will be handled as a separate task.

[mpg]

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