Use wxBG_STYLE_PAINT for wxGridWindow

This is more explicit, efficient and simpler than defining an empty
wxEVT_ERASE_BACKGROUND handler, which is not needed any longer.

src/generic/grid.cpp

@@ -1907,7 +1907,6 @@ wxBEGIN_EVENT_TABLE( wxGridWindow, wxGridSubwindow )
     EVT_CHAR( wxGridWindow::OnChar )
     EVT_SET_FOCUS( wxGridWindow::OnFocus )
     EVT_KILL_FOCUS( wxGridWindow::OnFocus )
-    EVT_ERASE_BACKGROUND( wxGridWindow::OnEraseBackground )
 wxEND_EVENT_TABLE()
 
 void wxGridWindow::OnPaint( wxPaintEvent &WXUNUSED(event) )
@@ -2281,10 +2280,6 @@ void wxGridWindow::OnChar( wxKeyEvent& event )
         event.Skip();
 }
 
-void wxGridWindow::OnEraseBackground( wxEraseEvent& WXUNUSED(event) )
-{
-}
-
 void wxGridWindow::OnFocus(wxFocusEvent& event)
 {
     // and if we have any selection, it has to be repainted, because it