Remove testing of mbedtls_ssl_conf_own_cert(NULL)

A future commit will test it on its own instead of as part of every positive
test.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/src/test_helpers/ssl_helpers.c b/tests/src/test_helpers/ssl_helpers.c
index dc34892..f5a8412 100644
--- a/tests/src/test_helpers/ssl_helpers.c
+++ b/tests/src/test_helpers/ssl_helpers.c
@@ -719,15 +719,6 @@
     ret = mbedtls_ssl_conf_own_cert(&(ep->conf), ep->cert,
                                     ep->pkey);
     TEST_EQUAL(ret, 0);
-    TEST_ASSERT(ep->conf.key_cert != NULL);
-
-    ret = mbedtls_ssl_conf_own_cert(&(ep->conf), NULL, NULL);
-    TEST_EQUAL(ret, 0);
-    TEST_ASSERT(ep->conf.key_cert == NULL);
-
-    ret = mbedtls_ssl_conf_own_cert(&(ep->conf), ep->cert,
-                                    ep->pkey);
-    TEST_EQUAL(ret, 0);
 
     ok = 1;