remove a bogus XXX note

lib/commands/pin-rpc.js

@@ -499,7 +499,10 @@ Pinning.loadChannelPins = function (Env) {
         surplus: 0,
         pinned: 0,
         duplicated: 0,
-        users: 0, // XXX useful for admin panel ?
+        // in theory we could use this number for the admin panel
+        // but we'd have to keep updating it whenever a new pin log
+        // was created or deleted. In practice it's probably not worth the trouble
+        users: 0,
     };
 
     const handler = function (ref, safeKey, pinned) {