Remove Executor#unregisterWorkQueue

We use ScheduledThreadPoolExecutor#terminated to manage cleanup of
WorkQueue. Callers of unregisterWorkQueue should just delete the call.

Change-Id: I0208c8643bd71eb287ff553f80e7ffe547f3ffbe

gerrit-server/src/main/java/com/google/gerrit/server/git/WorkQueue.java

@@ -204,7 +204,9 @@ public class WorkQueue {
       queueName = prefix;
     }
 
-    public void unregisterWorkQueue() {
+    @Override
+    protected void terminated() {
+      super.terminated();
       queues.remove(this);
     }