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.
cleaner. For sure. Easier... Hmmm... Actually creating a `/config/foo.backup/` would be easier. If everyone is OK with that, I'd prefer that solution.