finally remove long-deprecated 'Client' initializer

lib/mixpanel-node.js

@@ -470,9 +470,5 @@ var create_client = function(token, config) {
 
 // module exporting
 module.exports = {
-    Client: function(token) {
-        console.warn("The function `Client(token)` is deprecated.  It is now called `init(token)`.");
-        return create_client(token);
-    },
-    init: create_client
+    init: create_client,
 };