ssl_client: remove RNG check in `write_client_hello`

RNG check is added in ssl_conf_check when calling mbedtls_ssl_setup,
so there is no need to check it again.

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/library/ssl_client.c b/library/ssl_client.c
index 55fe352..bef7fc0 100644
--- a/library/ssl_client.c
+++ b/library/ssl_client.c
@@ -764,11 +764,6 @@
           MBEDTLS_SSL_SESSION_TICKETS &&
           MBEDTLS_HAVE_TIME */
 
-    if (ssl->conf->f_rng == NULL) {
-        MBEDTLS_SSL_DEBUG_MSG(1, ("no RNG provided"));
-        return MBEDTLS_ERR_SSL_NO_RNG;
-    }
-
     /* Bet on the highest configured version if we are not in a TLS 1.2
      * renegotiation or session resumption.
      */