Only require a ServerStatistics message when forwarding in ImageCallback

We were waiting for a ServerStatistics message when not forwarding.
But, there was/will never be one because it wasn't logged.  We aren't
going to use the data anyways, so just ignore it.

Change-Id: Ic1ce1a34d68fccb27a266149251fad0d024463bf
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/y2020/vision/charuco_lib.cc b/y2020/vision/charuco_lib.cc
index 77e1ba9..856c560 100644
--- a/y2020/vision/charuco_lib.cc
+++ b/y2020/vision/charuco_lib.cc
@@ -102,13 +102,13 @@
     const monotonic_clock::time_point eof_source_node =
         monotonic_clock::time_point(
             chrono::nanoseconds(image.monotonic_timestamp_ns()));
-    server_fetcher_.Fetch();
-    if (!server_fetcher_.get()) {
-      return;
-    }
-
     chrono::nanoseconds offset{0};
     if (source_node_ != event_loop_->node()) {
+      server_fetcher_.Fetch();
+      if (!server_fetcher_.get()) {
+        return;
+      }
+
       // If we are viewing this image from another node, convert to our
       // monotonic clock.
       const aos::message_bridge::ServerConnection *server_connection = nullptr;