Remove unused boolean from GemValue (#25)

Author: hduelme

api/src/main/java/org/mapstruct/tools/gem/GemValue.java

@@ -180,7 +180,6 @@ private ValueAnnotationValueListVisitor(Function<V, R> valueMapper) {
 
         @Override
         public List<R> visitArray(List<? extends AnnotationValue> vals, Class<V> vClass) {
-            boolean valid = true;
             List<R> values = new ArrayList<>( vals.size() );
             for ( AnnotationValue val : vals ) {
                 R value = val.accept( arrayVisitor, vClass );