PMD: Avoid modifiers which are implied by the context.

src/main/java/org/apache/commons/collections4/bidimap/TreeBidiMap.java

@@ -80,7 +80,7 @@ import org.apache.commons.collections4.keyvalue.UnmodifiableMapEntry;
 public class TreeBidiMap<K extends Comparable<K>, V extends Comparable<V>>
     implements OrderedBidiMap<K, V>, Serializable {
 
-    static enum DataElement {
+    enum DataElement {
         KEY("key"), VALUE("value");
 
         private final String description;

src/main/java/org/apache/commons/collections4/comparators/FixedOrderComparator.java

@@ -55,7 +55,7 @@ public class FixedOrderComparator<T> implements Comparator<T>, Serializable {
      * Unknown object behavior enum.
      * @since 4.0
      */
-    public static enum UnknownObjectBehavior {
+    public enum UnknownObjectBehavior {
         BEFORE, AFTER, EXCEPTION;
     }
 

src/main/java/org/apache/commons/collections4/map/AbstractReferenceMap.java

@@ -86,7 +86,7 @@ public abstract class AbstractReferenceMap<K, V> extends AbstractHashedMap<K, V>
     /**
      * Reference type enum.
      */
-    public static enum ReferenceStrength {
+    public enum ReferenceStrength {
         HARD(0), SOFT(1), WEAK(2);
 
         /** value */