Delete the WorkHandler class.

All work is still performed on the given workThread, just without the
need for a custom Handler class.

Bug: 68030053
Test: runtest --path
    frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java

Change-Id: Id4509821e51a1a8e1b171710b33b83212f2a506a