unit-tests: Remove 768 bits RSA gen test

Botan only allows RSA generating keys >= 1,024 bits, which makes
the RSA test suite fail. It is questionable whether it makes
sense to test 768 bit RSA keys anymore. They are too weak
from today's perspective anyway.

src/libstrongswan/tests/suites/test_rsa.c

@@ -146,7 +146,7 @@ static void test_bad_sigs(public_key_t *pubkey)
  * RSA key sizes to test
  */
 static int key_sizes[] = {
-	768, 1024, 1536, 2048, 3072, 4096,
+	1024, 1536, 2048, 3072, 4096,
 };
 
 START_TEST(test_gen)