Kernel: Remove ioctl for getting a socket peer's PID.

Now that everything is nice and mature, the WindowServer can just use the
client PID it receives in the Greeting message, and we can get rid of this
hacky ioctl. :^)

Servers/WindowServer/WSMessageLoop.cpp

@@ -257,6 +257,9 @@ void WSMessageLoop::on_receive_from_client(int client_id, const WSAPI_ClientMess
 
     WSClientConnection& client = *WSClientConnection::from_client_id(client_id);
     switch (message.type) {
+    case WSAPI_ClientMessage::Type::Greeting:
+        client.set_client_pid(message.greeting.client_pid);
+        break;
     case WSAPI_ClientMessage::Type::CreateMenubar:
         post_message(client, make<WSAPICreateMenubarRequest>(client_id));
         break;