Remove obsolete classes

Remove below classes
- CategoryFragment
- TopLevelPickerActivity
- IndividualPickerActivity

Bug: 198711415
Test: Build and run standalone WallpaperPicker2 project
Change-Id: I087d7fc28617b284b9b2a8e66823529d8cc8856f
diff --git a/src/com/android/wallpaper/picker/CategorySelectorFragment.java b/src/com/android/wallpaper/picker/CategorySelectorFragment.java
index a611d48..f2b1157 100644
--- a/src/com/android/wallpaper/picker/CategorySelectorFragment.java
+++ b/src/com/android/wallpaper/picker/CategorySelectorFragment.java
@@ -243,8 +243,7 @@
     }
 
     /**
-     * Notifies the CategoryFragment that no further categories are expected so it may hide
-     * the loading indicator.
+     * Notifies that no further categories are expected so it may hide the loading indicator.
      */
     void doneFetchingCategories() {
         if (mAwaitingCategories) {