Revert 1de107c037 and remove the focus
event handler which duplicated and interfered with the handler inherited
from this class to make sure that we get wxEVT_{SET,KILL}_FOCUS for the
objects of this class.
Unfortunately these events now come in pairs, due to an extra artificial
event generated by wxComboBoxExtraInputHandler::OnFocus(), which is
still wrong -- but arguably less wrong and more useful than not sending
them at all.
97e217e175