Stop referring to WorkQueue.Executor.

Change-Id: I65db402e8fdc32dff829385e542b4cfab9f441d9

src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java
index e1352b4..bc7e5eb 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookQueue.java
@@ -19,11 +19,12 @@
 import com.google.inject.Inject;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.concurrent.ScheduledExecutorService;
 
 class HookQueue implements LifecycleListener {
   private final WorkQueue workQueue;
 
-  private WorkQueue.Executor queue;
+  private ScheduledExecutorService queue;
 
   @Inject
   HookQueue(WorkQueue queue) {