Delete the MainHandler and remaining code.

As of ag/3516264 this is no longer required. Next CL will remove the
double lists and pending notification structure.

Bug: 37674366
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java
1. Manual testing of WifiSettings for visual jank
2. Start up and confirmation of SetupWizard wifi picker.

Change-Id: I3f2c649375f245295a00177f805d6d3e883fe656