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.
@uboness What about sub dirs? 

If a plugin has for the first time:

```
/config/foo/bar/file1.yml
```

And 

```
/config/foo/bar/file1.yml
/config/foo/bar/file2.yml
```

Should I use `Files.walkFileTree` to go in each subdir/file or just take care of the first level of files?