playground: remove stale TODO about exit code

This feature was implemented.

Change-Id: I6b0d9b121f2ccf517bb0f7c7be7c2aa602456acc
Reviewed-on: https://go-review.googlesource.com/c/146617
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/sandbox.go b/sandbox.go
index 5988085..7fb6964 100644
--- a/sandbox.go
+++ b/sandbox.go
@@ -4,7 +4,6 @@
 
 // TODO(andybons): add logging
 // TODO(andybons): restrict memory use
-// TODO(andybons): send exit code to user
 
 package main