Don't force themed background for wxToolBar under MSW, just use the default.

Don't force themed background for wxToolBar under MSW, just use the default.

Native toolbars don't have themed background under Vista/7 and don't look good
with rebar background (which is the same one as used for the menus) as
toolbars don't appear on the top of the window.

Erasing background ourselves also results in flicker and display problems and
makes the code much more complicated. Simply don't do this to avoid all the
problems.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62931 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775