Don't cache HDC used by wxPaintDCEx in wxMSW.

Don't cache HDC used by wxPaintDCEx in wxMSW.

This avoids the problem with mistakenly using wrong HDC in wxBitmapComboBox
code which was due to assuming that we can only ever have one paint HDC for
the given window -- while in wxBitmapComboBox case we are passed different
HDCs for the same window via WM_DRAWITEM.

Instead of fixing the cache, just don't use it at all for wxPaintDCEx as we
don't gain anything from doing it anyhow.

Closes #14842.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73625 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775