Fix rare test failures of TestIndexSorting.

Sometimes the final merge might not require sorting depending on the merge
policy configuration.

lucene/core/src/test/org/apache/lucene/index/TestIndexSorting.java

@@ -188,6 +188,7 @@ public class TestIndexSorting extends LuceneTestCase {
     assertTrue(codec.numCalls > 0);
 
     // merge sort is needed
+    w.deleteAll();
     codec.numCalls = 0;
     codec.needsIndexSort = true;
     for (int i = 10; i >= 0; i--) {