Delete the outdated button widget demo code.

- "rv" is not recommended and "glance" is the preferred widget framework.
- Sizing on those widgets is off. The new canonical layouts are recommended.

Author: shamalip

samples/user-interface/appwidgets/src/main/AndroidManifest.xml

@@ -221,18 +221,6 @@
                 android:name="android.appwidget.provider"
                 android:resource="@xml/app_widget_weather_forecast_glance" />
         </receiver>
-        <receiver
-            android:name=".glance.buttons.ButtonsGlanceWidgetReceiver"
-            android:enabled="@bool/glance_appwidget_available"
-            android:exported="false">
-            <intent-filter>
-                <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
-            </intent-filter>
-
-            <meta-data
-                android:name="android.appwidget.provider"
-                android:resource="@xml/app_widget_buttons_glance" />
-        </receiver>
         <receiver
             android:name=".glance.image.ImageGlanceWidgetReceiver"
             android:enabled="@bool/glance_appwidget_available"