Remove old screenshots notification channel cleanup code.

This saves hundreds of ms of binder calls when SystemUI
starts up (to INotificationManager.getNotificationChannel
and INotificationManager.deleteNotificationChannel).

Fixes: 201541113
Test: (boot device)
Test: (test secondary users, user switching)
Test: atest SystemUITests
Change-Id: Ie1d011581c49878911e8ddb46121fa95c24339ba