Simplify definition of TLS 1.3 MD max size.

Actually this macro is never used in parts that depend on USE_PSA, so
it's always using PSA.

Currently the macro seems a bit redundant, but:
- since it's public we can't remove it;
- and there are plans in the future to make it more precise (actually
the largest hash that matters for TLS 1.3 is SHA-384 now).

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 1e0220a..3811fd2 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -629,11 +629,7 @@
 
 #define MBEDTLS_PREMASTER_SIZE     sizeof( union mbedtls_ssl_premaster_secret )
 
-#if defined(MBEDTLS_USE_PSA_CRYPTO)
 #define MBEDTLS_TLS1_3_MD_MAX_SIZE         PSA_HASH_MAX_SIZE
-#else
-#define MBEDTLS_TLS1_3_MD_MAX_SIZE         MBEDTLS_MD_MAX_SIZE
-#endif /* MBEDTLS_USE_PSA_CRYPTO */
 
 
 /* Length in number of bytes of the TLS sequence number */