Revert "Merge branch 'windowing-system' into 'master'"

This reverts merge request !541 (merged)

This fix was incorrect. The macro does exist and OSG always falls back to X11 in this macro even if the windowing system is set to "None".

The correct fix has been merged into OSG itself, including the 3.6 branch. Reverting the commit here to still be compatible with older OSG versions.