Don't draw the mysterious black rectangle in the erase sample.

Don't draw the mysterious black rectangle in the erase sample.

The black rectangle drawn in EVT_PAINT handler in the sample prevented the
custom background from showing through the wxStaticBitmap. Maybe it was
initially done intentionally but this makes the sample look broken so just
don't do this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67265 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775