fix: remove undefined navigationOrder field from the JSON report

This commit remove the `navigationOrder` field from the JSON report, as this was no longer set by EPUBCheck.

Navigation order consistency is better checked manually or with help of external tools.

See #1515

src/main/java/com/adobe/epubcheck/reporting/ItemMetadata.java

@@ -31,8 +31,6 @@ public class ItemMetadata implements Comparable<ItemMetadata>
   @JsonProperty
   private boolean isLinear;
   @JsonProperty
-  private Integer navigationOrder = null;
-  @JsonProperty
   private Boolean isFixedFormat = null;
   @JsonProperty
   private boolean isScripted;
@@ -170,9 +168,6 @@ public void handleInfo(FeatureEnum feature, String value)
       case RENDITION_SPREAD:
         this.renditionSpread = value;
         break;
-      case NAVIGATION_ORDER:
-        this.navigationOrder = Integer.parseInt(value.trim());
-        break;
       default:
         //System.err.printf("unhandled info message feature: found '%s' with value '%s'", feature.toString(), value != null ? value : "null");
         break;

src/main/java/com/adobe/epubcheck/util/FeatureEnum.java

@@ -65,7 +65,6 @@ public enum FeatureEnum
   MEDIA_OVERLAYS_ACTIVE_CLASS("media:active-class"),
   MEDIA_OVERLAYS_PLAYBACK_ACTIVE_CLASS("media:playback-active-class"),
   MODIFIED_DATE("modification date"),
-  NAVIGATION_ORDER("navigation order"),
   PAGE_BREAK("epub:page-break"),
   PAGE_LIST("epub:page-list"),
   PAGES_COUNT("pages count"),