[Wi-Fi] Replace WifiTracker with WifiPickerTracker in WifiSettings

WifiSettings uses WifiTracker in SettingsLib while WifiSettings2
uses WifiPickerTracker in WifiSettingsLib.

1. Remove WifiSettings.
2. Rename WifiSettings2 to WifiSettings.
3. Remove the files only used in the removed WifiSettings.
   (Saved networks files are not included)

Bug: 152571756
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.wifi
      atest WifiSettingsUiTest
Change-Id: I800b434c8049121db115cff87d51e164e4529999