Clean ProxyPreferenceController

This is a controller without UI.

Bug: 311604902
Test: manual - on Network & internet page
Change-Id: Id88709b5c3f64ef7f6b441260b4b88ce7a9a5847

src/com/android/settings/network/NetworkDashboardFragment.java

@@ -100,7 +100,6 @@ public class NetworkDashboardFragment extends DashboardFragment implements
         controllers.add(new MobileNetworkSummaryController(context, lifecycle, lifecycleOwner));
         controllers.add(new TetherPreferenceController(context, lifecycle));
         controllers.add(vpnPreferenceController);
-        controllers.add(new ProxyPreferenceController(context));
         if (internetPreferenceController != null) {
             controllers.add(internetPreferenceController);
         }