Cleanup: UI: Remove unused asset shelf settings pointers

The asset shelf settings struct wasn't used for listbase storage, so
these listbase pointers were unused. They were added unintentionally.

source/blender/editors/asset/intern/asset_shelf_settings.cc

@@ -44,8 +44,6 @@ AssetShelfSettings &AssetShelfSettings::operator=(const AssetShelfSettings &othe
   /* Start with a shallow copy. */
   memcpy(this, &other, sizeof(AssetShelfSettings));
 
-  next = prev = nullptr;
-
   if (active_catalog_path) {
     active_catalog_path = BLI_strdup(other.active_catalog_path);
   }

source/blender/makesdna/DNA_screen_types.h

@@ -795,8 +795,6 @@ enum {
 };
 
 typedef struct AssetShelfSettings {
-  struct AssetShelfSettings *next, *prev;
-
   AssetLibraryReference asset_library_reference;
 
   ListBase enabled_catalog_paths; /* #AssetCatalogPathLink */