Remove option to disable ipv6

It has been default for a while, is the only tested option,
and will only get more common.

src/fe-text/fe-text.c

@@ -111,9 +111,6 @@ fe_new_window (struct session *sess, int focus)
 #endif
 #ifdef USE_OPENSSL
 	"OpenSSL "
-#endif
-#ifdef USE_IPV6
-	"IPv6"
 #endif
 	"\n\n", 0, FALSE);
 	fflush (stdout);