Minor DocValuesDocReader cleanup (#123619)

The instance members can be private, all final, and SearchLookup is unused.

server/src/main/java/org/elasticsearch/script/DocValuesDocReader.java

@@ -27,16 +27,13 @@ import java.util.stream.Stream;
  */
 public class DocValuesDocReader implements DocReader, LeafReaderContextSupplier {
 
-    protected final SearchLookup searchLookup;
-
     // provide access to the leaf context reader for expressions
-    protected final LeafReaderContext leafReaderContext;
+    private final LeafReaderContext leafReaderContext;
 
     /** A leaf lookup for the bound segment this proxy will operate on. */
-    protected LeafSearchLookup leafSearchLookup;
+    private final LeafSearchLookup leafSearchLookup;
 
     public DocValuesDocReader(SearchLookup searchLookup, LeafReaderContext leafContext) {
-        this.searchLookup = searchLookup;
         this.leafReaderContext = leafContext;
         this.leafSearchLookup = searchLookup.getLeafSearchLookup(leafReaderContext);
     }