This PR removes unused Win32 code that we've had for about 13 years,
hidden behind a define that we've never enabled. The intent of this
optional code was to allow showing Blender windows as transparent,
something that looked pretty cool back then. The main method used was
an API call of DwmEnableBlurBehindWindow, but this function has not
done anything since Windows 8 because styles and tastes have changed.
Pull Request: https://projects.blender.org/blender/blender/pulls/137620
2e1a12905d