Remove the fallback AddSupervisedUserActivity.

Rollback the changes done in ag/16388725 and ag/16709333

Bug: 209659998
Test: croot && make RunSettingsRoboTests -j40 ROBOTEST_FILTER="com.android.settings.users.UserSettingsTest"

Change-Id: I75b85df50a3caff55375ad261a97225377644a38

src/com/android/settings/users/UserSettings.java

@@ -514,13 +514,6 @@ public class UserSettings extends SettingsPreferenceFragment
                 .setPackage(mConfigSupervisedUserCreationPackage)
                 .addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
 
-        // TODO(b/209659998): [to-be-removed] fallback activity for supervised user creation.
-        if (getActivity().getPackageManager().resolveActivity(intent, 0) == null) {
-            intent
-                .setClass(getContext(), AddSupervisedUserActivity.class)
-                .setPackage(null);
-        }
-
         startActivity(intent);
     }