Remove redundant assertion from test

It's already asserted in other tests, and is causing flakes.

Change-Id: I02ea5cf200e535410e7897a51caf1c98d1682068
Fixes: 70530199
Test: robotest

tests/robotests/src/com/android/settings/wifi/tether/WifiTetherPreferenceControllerTest.java

@@ -137,7 +137,6 @@ public class WifiTetherPreferenceControllerTest {
         mController.displayPreference(mScreen);
         mLifecycle.handleLifecycleEvent(ON_START);
 
-        assertThat(ShadowWifiTetherSwitchBarController.onStartCalled).isTrue();
         verify(mContext).registerReceiver(eq(receiver), any(IntentFilter.class));
         verify(pref).setChecked(false);
     }