LibIPC: Stop exchanging client/server PIDs in greeting handshake

The PIDs were used for sharing shbufs between processes, but now that
we have migrated to file descriptor passing, we no longer need to know
the PID of the other side.

Userland/Services/ImageDecoder/ImageDecoderServer.ipc

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