Plugins: Disable support for config dir

When removing and installing again the plugin all configuration files will be removed in `config/pluginname` dir.
This is bad as users may have set and added specific configuration files.

We don't have yet an upgrade command which could handle this nicely.

So we remove that code for now and we will implement an upgrade method in another PR.

Related to  #5064.
I still think skipping is better tbh... otherwise the users will be more confused on how to merge the old one with the new one. I don't think the file formats will change that much... but when they do, we'll simply be able to tell them what to change in the files they already configured in order to be compatible with the new version (not sure that will even be needed, we could perhaps support both formats in the code and make it transparent).