Save O(1s) of CPU time in FieldSortIT (#118146)

I see this `toString` take ~2s of hot CPU time in some test runs which isn't entirely surprising.
Rather than optimize this in some form, just dropping the string here which aligns the thing with
other tests anyway.

Author: original-brownbear

server/src/internalClusterTest/java/org/elasticsearch/search/sort/FieldSortIT.java

@@ -202,7 +202,6 @@ public void testIssue6614() throws InterruptedException {
                         response -> {
                             for (int j = 0; j < response.getHits().getHits().length; j++) {
                                 assertThat(
-                                    response.toString() + "\n vs. \n" + allDocsResponse.toString(),
                                     response.getHits().getHits()[j].getId(),
                                     equalTo(allDocsResponse.getHits().getHits()[j].getId())
                                 );