CMake: remove use of removed kcoreaddons_desktop_to_json macro

That macro has been deprecated in KF5 and is now removed in KF6 The deprecation warning in KF5 mentions the files should be changed to json in source using the "desktoptojson" executable but this particular file is generated during building so it's a bit harder to do.

I'm not sure why the json output is needed, if it is still at all. I suppose if it is required we should call the desktoptojson executable directly?