Remove Lucene.isEmpty(DocIdSet).

After the Filter ban, this method is not used anymore.

core/src/main/java/org/elasticsearch/common/lucene/Lucene.java

@@ -717,13 +717,6 @@ public class Lucene {
         }
     }
 
-    /**
-     * Is it an empty {@link DocIdSet}?
-     */
-    public static boolean isEmpty(@Nullable DocIdSet set) {
-        return set == null || set == DocIdSet.EMPTY;
-    }
-
     /**
      * Given a {@link Scorer}, return a {@link Bits} instance that will match
      * all documents contained in the set. Note that the returned {@link Bits}