LibIPC: Stop sending client ID to clients

The client ID is not useful to normal clients anymore, so stop telling
everyone what their ID is.

Userland/Services/NotificationServer/NotificationServer.ipc

@@ -1,7 +1,7 @@
 endpoint NotificationServer = 95
 {
     // Basic protocol
-    Greet() => (i32 client_id)
+    Greet() => ()
 
     ShowNotification([UTF8] String text, [UTF8] String title, Gfx::ShareableBitmap icon) => ()
 }