Remove unreachable logic after error(...) is called.

The function error(...) always throws so there is no need to return separately
or have an else branch.

src/crypto.js

@@ -570,7 +570,6 @@ var CipherTransformFactory = (function CipherTransformFactoryClosure() {
       };
     }
     error('Unknown crypto method');
-    return null;
   }
 
   CipherTransformFactory.prototype = {