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/ImageDecoder/ImageDecoderServer.ipc

@@ -1,6 +1,6 @@
 endpoint ImageDecoderServer = 7001
 {
-    Greet() => (i32 client_id)
+    Greet() => ()
 
     DecodeImage(Core::AnonymousBuffer data) => (bool is_animated, u32 loop_count, Vector<Gfx::ShareableBitmap> bitmaps, Vector<u32> durations)
 }