HADOOP-16851. Removed unused import in Configuration

Contributed by Jan Hentschel.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/conf/Configuration.java

@@ -51,7 +51,6 @@ import java.util.Enumeration;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
-import java.util.LinkedList;
 import java.util.List;
 import java.util.ListIterator;
 import java.util.Map;