Remove LayoutItemPortal.get/setNavigationTable().

Remove LayoutItemPortal.get/setNavigationTable().

* src/main/java/org/glom/web/shared/libglom/layout/LayoutItemPortal.java:
Remove get/setNavigationTable(), which is only a cache, because it is not
used, and does not need to be used, because that decision should be made on
the server.
* src/main/java/org/glom/web/server/ConfiguredDocument.java:
updatePortalsExtras():
* src/main/java/org/glom/web/server/database/RelatedListNavigation.java:
getNavigationRecord():
* src/test/java/org/glom/web/server/libglom/LayoutItemPortalDeepCloneTest.java:
Adapted.