Clean up split system user code in packages/apps/Settings/

Remove legacy code belonged to Andrometa project. Split system user
feature is always turned off now.

Bug: 137101239
Test: build, sync and reboot
Change-Id: Ieb84a6e131383dcd263a5d9b29a49dc64d905fe0

tests/robotests/src/com/android/settings/wifi/WifiConfigControllerTest.java

@@ -411,11 +411,6 @@ public class WifiConfigControllerTest {
             super(parent, view, accessPoint, mode, wifiManager);
         }
 
-        @Override
-        boolean isSplitSystemUser() {
-            return false;
-        }
-
         @Override
         KeyStore getKeyStore() { return mKeyStore; }
     }