Remove  additional unused no rng case

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index edcc14d..89de959 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -550,14 +550,7 @@
         TEST_ASSERT(p < end);
     }
 
-    // When using PSA crypto, RNG isn't controllable, result length isn't
-    // deterministic over multiple runs, removing a single byte isn't enough to
-    // go into the MBEDTLS_ERR_ASN1_BUF_TOO_SMALL error case
-    if (issuer_key_type != MBEDTLS_PK_RSA) {
-        der_len /= 2;
-    } else {
-        der_len -= 1;
-    }
+    der_len -= 1;
 
     ret = mbedtls_x509write_crt_der(&crt, buf, (size_t) (der_len));
     TEST_ASSERT(ret == MBEDTLS_ERR_ASN1_BUF_TOO_SMALL);