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.
Would it be easier to copy the old config to somewhere else, then replace it rather than trying to mix adding new files while keeping old files? I feel like this will be very confusing for users, especially if `file1.yml` had changes coming from both directions.