Stop using WorkQueue#unregisterWorkQueue.

Change-Id: I7832acee3d901457330d240200212bef04da32ac
diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java b/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
index 4b22a45..91fd4f0 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/Destination.java
@@ -217,7 +217,6 @@
         repLog.warn(String.format("Cancelling replication event %s", r));
       }
       cnt = pool.shutdownNow().size();
-      pool.unregisterWorkQueue();
       pool = null;
     }
     return cnt;