Remove MobileNetworkSettingsTest#onAttach

The MobilieNetworkActivity has MobileNetworkSettings fragment. The
MobileNetworkActivityTest do testcases for activity stability.
The activity's launch test successful is equals the fragment doesn't
crash, so the activity's testcases cover
MobileNetworkSettingsTest#onAttach.

Bug: 147464840
Test: build pass
Change-Id: Idbabb846c40fd117f22a39b86fd6628b7d8c5b1b

tests/robotests/src/com/android/settings/network/telephony/MobileNetworkSettingsTest.java

@@ -44,7 +44,6 @@ import com.android.settings.widget.EntityHeaderController;
 import com.android.settingslib.core.AbstractPreferenceController;
 
 import org.junit.Before;
-import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.mockito.Mock;
@@ -91,13 +90,6 @@ public class MobileNetworkSettingsTest {
                 mNetworkPolicyManager);
     }
 
-    @Test
-    @Ignore
-    public void onAttach_noCrash() {
-        mFragment.onAttach(mContext);
-    }
-
-
     @Test
     public void createPreferenceControllers_createsDataUsageSummaryController() {
         final List<AbstractPreferenceController> controllers =