Remove unused imports from Netty4Utils

This commit removes two unused imports from Netty4Utils that were
leftover from a previous change.

modules/transport-netty4/src/main/java/org/elasticsearch/transport/netty4/Netty4Utils.java

@@ -32,8 +32,6 @@ import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.logging.ESLoggerFactory;
 
 import java.io.IOException;
-import java.io.PrintWriter;
-import java.io.StringWriter;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;