ssl: remove OpenSSL::ExtConfig::TLS_DH_anon_WITH_AES_256_GCM_SHA384

The constant was initially introduced just to skip test cases that do
not work with old versions without AES-GCM cipher suites support
(< 1.0.1).

However, the value of the constant has been always `false' because the
macro TLS_DH_anon_WITH_AES_256_GCM_SHA384 does not exist in any version
of OpenSSL.

We no longer use it as of commit c9d1659 ("test/utils: remove
use_anon_cipher option from SSLTestCase#start_server", 2016-09-06).
Let's just remove the constant.

Author: rhenium

ext/openssl/ossl_ssl.c

@@ -2453,12 +2453,6 @@ Init_ossl_ssl(void)
 
     rb_define_const(mSSLExtConfig, "HAVE_TLSEXT_HOST_NAME", Qtrue);
 
-#ifdef TLS_DH_anon_WITH_AES_256_GCM_SHA384
-    rb_define_const(mSSLExtConfig, "TLS_DH_anon_WITH_AES_256_GCM_SHA384", Qtrue);
-#else
-    rb_define_const(mSSLExtConfig, "TLS_DH_anon_WITH_AES_256_GCM_SHA384", Qfalse);
-#endif
-
     /*
      * A callback invoked whenever a new handshake is initiated. May be used
      * to disable renegotiation entirely.