Fixed findbugs finding

Author: S1artie

de.gebit.integrity.jenkins/src/main/java/de/gebit/integrity/IntegrityHistory.java

@@ -193,11 +193,8 @@ public static class MyStackedAreaRenderer extends StackedAreaRenderer2 {
 		 */
 		private static final long serialVersionUID = 2284582320567360791L;
 
-		private CategoryDataset dataset;
-
 		public MyStackedAreaRenderer(CategoryDataset aDataset) {
 			super();
-			dataset = aDataset;
 		}
 
 		@Override