Use Runnable instead of Callable in MailReceiver

Callable fails with this exception:
java.lang.UnsupportedOperationException: Callable not implemented

Other parts in Gerrit use Runnable. Using Runnable here as well fixed
the issue.

Change-Id: I36c96c8408a7295b06217f2d6df18e8253b0e1c8

gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/MailReceiver.java

@@ -27,8 +27,6 @@ import java.util.List;
 import java.util.Set;
 import java.util.Timer;
 import java.util.TimerTask;
-import java.util.concurrent.Callable;
-import java.util.concurrent.Future;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -124,7 +122,9 @@ public abstract class MailReceiver implements LifecycleListener {
   protected void dispatchMailProcessor(List<MailMessage> messages, boolean async) {
     for (MailMessage m : messages) {
       if (async) {
-        Callable<?> task =
+        workQueue
+            .getDefaultQueue()
+            .submit(
                 () -> {
                   try {
                     mailProcessor.process(m);
@@ -132,10 +132,7 @@ public abstract class MailReceiver implements LifecycleListener {
                   } catch (OrmException e) {
                     log.error("Mail: Can't process message " + m.id() + " . Won't delete.", e);
                   }
-              return null;
+                });
-            };
-        @SuppressWarnings("unused")
-        Future<?> possiblyIgnoredError = workQueue.getDefaultQueue().submit(task);
       } else {
         // Synchronous processing is used only in tests.
         try {