Assign unused Future return value to a variable

Error Prone plans to make this pattern into a compile error; this avoids
future breakage.

Change-Id: I6a023f22f472941854a6729f6450b9b70a020069

src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java
index 24c9336..1a80875 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewnotes/RefUpdateListener.java
@@ -86,7 +86,8 @@
           }
         };
     if (async) {
-      workQueue.getDefaultQueue().submit(task);
+      @SuppressWarnings("unused") // No assurance this completes.
+      Future<?> possiblyIgnoredError = workQueue.getDefaultQueue().submit(task);
     } else {
       task.run();
     }