Remove hardcoded cacheExtent to use Flutter's default value

mobile/apps/photos/lib/ui/viewer/gallery/gallery.dart

@@ -604,7 +604,6 @@ class GalleryState extends State<Gallery> {
                           ? const NeverScrollableScrollPhysics()
                           : const ExponentialBouncingScrollPhysics(),
                       controller: _scrollController,
-                      cacheExtent: 1000,
                       slivers: [
                         SliverToBoxAdapter(
                           child: SizeChangedLayoutNotifier(
@@ -642,25 +641,6 @@ class GalleryState extends State<Gallery> {
             ),
     );
   }
-
-  double get galleryCacheExtent {
-    final int photoGridSize = localSettings.getPhotoGridSize();
-    switch (photoGridSize) {
-      case 2:
-      case 3:
-        return 1000;
-      case 4:
-        return 850;
-      case 5:
-        return 600;
-      case 6:
-        return 300;
-      default:
-        throw StateError(
-          'Invalid photo grid size configuration: $photoGridSize',
-        );
-    }
-  }
 }
 
 class PinnedGroupHeader extends StatefulWidget {