Remove MBEDTLS_AES_ALT

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/tf-psa-crypto/drivers/builtin/src/gcm.c b/tf-psa-crypto/drivers/builtin/src/gcm.c
index 5dfac23..bf21943 100644
--- a/tf-psa-crypto/drivers/builtin/src/gcm.c
+++ b/tf-psa-crypto/drivers/builtin/src/gcm.c
@@ -1062,8 +1062,8 @@
                                      key_len);
             /*
              * AES-192 is an optional feature that may be unavailable when
-             * there is an alternative underlying implementation i.e. when
-             * MBEDTLS_AES_ALT is defined.
+             * there is an alternative underlying implementation such as a
+             * PSA driver.
              */
             if (ret == MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED && key_len == 192) {
                 mbedtls_printf("skipped\n");