Do not use TetherUtil.setWifiTethering

This functionality is replaced by ConnectivityManager.startTethering()
and ConnectivityManager.stopTethering().

Bug: 30147602
Test: Can toggle tethering on/off via all of settings, quick settings,
      and quick quick settings.
Change-Id: I4d190185c473237982b988f898e76d4997dc401c

src/com/android/settings/wifi/WifiApEnabler.java

@@ -110,16 +110,6 @@ public class WifiApEnabler {
         }
     }
 
-    public void setSoftapEnabled(boolean enable) {
-        if (TetherUtil.setWifiTethering(enable, mContext)) {
-            /* Disable here, enabled on receiving success broadcast */
-            mSwitch.setEnabled(false);
-        } else {
-            mSwitch.setSummary(R.string.wifi_error);
-        }
-
-    }
-
     public void updateConfigSummary(WifiConfiguration wifiConfig) {
         String s = mContext.getString(
                 com.android.internal.R.string.wifi_tether_configure_ssid_default);