Remove `KIWIX_DATA_DIR` about libkiwix

[mgautierfr]

Is not clear which feature it serves (seems useless to me)

It is a way to configure libkiwix and overide the resolution order of getDataDirectory

I'm again to instrument Kiwix Desktop with "hidden" variable environnements.

It is a libkiwix feature, which is used by kiwix-desktop

Is not documented (the best doc I have is kiwix/kiwix-desktop#694 (comment) from @veloman-yunkan )

Documented here : https://libkiwix.readthedocs.io/en/latest/api/function_tools_8h_1ab559be1bc8eca66c26901660949870c0.html

As a bad naming because only part of the data are stored in it

I would say that it would be to kiwix-desktop to always use the directory returned by kiwix::getDataDirectory unless specific config changed that (and fix kiwix-desktop if it is not the case (whatever we decide about KIWIX_DATA_DIR))

[kelson42]

@mgautierfr Thank you for the clarification, I have moved the issue. The documentation link is also very helpful to understand. I have created an issue to request a proper documentation of all local paths to save/load things in the Wiki kiwix/kiwix-desktop#1114

[kelson42]

All of this is Kiwix Desktop related and should be handled in Kiwix Desktop. The need is not clear to me why we should need to customize this from outside the app and it we would have then it would have to be done via a command line argument. On the top the whole Kiwix Desktop UI does not take care of this and continue to show the usual path. This "feature" has been transformed in a bug. We should just remove it.

Should be implemented within #1087