Fix wxOwnerDrawnComboBox background on with classic theme.

Fix wxOwnerDrawnComboBox background on with classic theme.

In wxMSW, if the classic theme was used, read-only wxOwnerDrawnComboBox
had gray background, but native look should be the same for both
editable and read-only combo boxes: white text entry-like rectangle.

Apparently, a similar problem existed with non-classic themes too
before, as there was a workaround in place. Fixed by moving the
workaround into code used by both themed and classic rendering.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71367 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775