Remove the test of erase background events from the toolbar sample.

Remove the test of erase background events from the toolbar sample.

Handling erase background events for toolbars never worked for the ports other
than wxMSW and now it doesn't work for MSW neither as we need to skip erasing
the background in WM_ERASEBKGND handler to avoid flicker which would result
from erasing the background twice, from there and from WM_PAINT handler which
must do it. So don't even try to test if it works, it's unlikely to be very
useful anyhow.

This reverts the change to this file in r62805.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62972 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775