Remove compilation option MBEDTLS_PSA_HAS_ITS_IO

MBEDTLS_PSA_HAS_ITS_IO is not really useful since it doesn't actually
enable anything except the entropy seed file support, which only
requires the ITS interface and not a native implemetation. Remove it.
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index 5dd4789..545dd4b 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -115,7 +115,7 @@
  *
  * \note This function is only available on the following platforms:
  * * If the compile-time options MBEDTLS_ENTROPY_NV_SEED and
- *   MBEDTLS_PSA_HAS_ITS_IO are both enabled. Note that you
+ *   MBEDTLS_PSA_CRYPTO_STORAGE_C are both enabled. Note that you
  *   must provide compatible implementations of mbedtls_nv_seed_read
  *   and mbedtls_nv_seed_write.
  * * In a client-server integration of PSA Cryptography, on the client side,