des.js: remove decrypt and simplify.

The decrypt functionality is never used so remove it. Also, we can
assume that we are always DES encrypting 16 characters which allows
several things to be simplified in DES.

Overall this removes about 80 lines of code.

include/rfb.js

@@ -602,17 +602,11 @@ function send_string(str) {
 }
 
 function genDES(password, challenge) {
-    var i, passwd, response;
-    passwd = [];
-    response = challenge.slice();
+    var i, passwd = [];
     for (i=0; i < password.length; i += 1) {
         passwd.push(password.charCodeAt(i));
     }
-
-    DES.setKeys(passwd);
-    DES.encrypt(response, 0, response, 0);
-    DES.encrypt(response, 8, response, 8);
-    return response;
+    return DES.encrypt(passwd, challenge);
 }
 
 function flushClient() {