Simplify favorites link JS

Make use of the "is favorited" data to determine if the post is favorited instead of relying on the entire list of favorites.

app/assets/javascripts/favorites.js

@@ -8,7 +8,6 @@
   }
 
   Danbooru.Favorite.hide_or_show_add_to_favorites_link = function() {
-    var favorites = Danbooru.meta("favorites");
     var current_user_id = Danbooru.meta("current-user-id");
     if (current_user_id == "") {
       $("#add-to-favorites").hide();
@@ -17,8 +16,7 @@
       $("#remove-fav-button").hide();
       return;
     }
-    var regexp = new RegExp("\\bfav:" + current_user_id + "\\b");
-    if ((favorites != undefined) && (favorites.match(regexp))) {
+    if ($("#image-container").length && $("#image-container").data("is-favorited") == true) {
       $("#add-to-favorites").hide();
       $("#add-fav-button").hide();
     } else {