Remove the TOOLKIT test from msvc-headers-setuo-h definition.

This is unnecessary now that TOOLKIT is back to being a constant again and
results in errors if GTK TOOLKIT is enabled.

Closes #14965.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74438 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775