This constant wasn't very useful as it contained the description of the OS on
the machine where the library was built, not the one on which the application
using it was running. It also wasn't used anywhere in wxWidgets and apparently
wasn't meant to be used outside of it.
Finally, putting the output of `uname -r` into it created problems with
creating reproducible builds as just a change in the kernel version changed
the build results.
Closes#17002.
