Fix PMD issue (unnecessary private modifier in enum constructor)

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

@@ -111,7 +111,7 @@ public abstract class AbstractReferenceMap<K, V> extends AbstractHashedMap<K, V>
             }
         }
 
-        private ReferenceStrength(final int value) {
+        ReferenceStrength(final int value) {
             this.value = value;
         }