Demote a needless warning to info.

Author: jannistsiroyannis

housekeeping/src/main/groovy/whelk/housekeeping/ImageLinker.java

@@ -288,7 +288,7 @@ private void linkImage(String instanceUri, String imageUri) {
         List<Map<String, String>> imageList = tempDoc.getImages();
         for (Map<String, String> image : imageList) {
             if (image.containsValue(imageUri)) {
-                logger.warn("{} already contains image {}; not linking", instanceId, imageUri);
+                logger.info("{} already contains image {}; not linking", instanceId, imageUri);
                 return;
             }
         }