Do not mark generated files as GENERATED

[tmadlener]

BEGINRELEASENOTES

• Remove the GENERATED property from generated files in CMake to avoid inconsistent removal of headers and source files with the clean target. Fixes #396

ENDRELEASENOTES

@andresailer do you have any better idea for this? There was quite some discussion already in #143 but nothing too specific about the GENERATED property.

@DraTeots could you give this a go and check whether this indeed solves your issue?

[DraTeots]

Tested. It fully fixes my issue. Thank you!!!

[tmadlener]

@jmcarcell just to make sure, this also doesn't break any of your workflows?

[jmcarcell]

It doesn't break anything for me because I don't use clean. But with cmake 3.25.2 I can't reproduce the original issue, in my case neither the headers nor the .cc files are deleted when running clean. Also using the version in this PR nothing is deleted. There is something weird about this because the documentation says that:

The Makefile Generators will remove GENERATED files during make clean.

so it should delete both headers and .cc files (master)?. I think that anyway it's better to remove this option since it doesn't say anything for other generators so maybe there is different behaviour for make and ninja for example