Make IdentifiedUser.clearStarredChanges return void

This method was always returning null. Easier to declare it returns
void instead of a Set<Change.Id> that is always null. Fortunately no
caller used the return value.

Change-Id: I198e0841cf2e9f07e2995700987518c1b99ac6f4

gerrit-server/src/main/java/com/google/gerrit/server/IdentifiedUser.java

@@ -341,12 +341,11 @@ public class IdentifiedUser extends CurrentUser {
     return starredChanges;
   }
 
-  public Set<Change.Id> clearStarredChanges() {
+  public void clearStarredChanges() {
     // Async query may have started before an update that the caller expects
     // to see the results of, so we can't trust it.
     abortStarredChanges();
     starredChanges = null;
-    return starredChanges;
   }
 
   public void asyncStarredChanges() {