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 don't get it. If we have another exception than IOException, I guess it will be raised and not catch, right? Do you mean we should have only:

```java
try {
  Files.copy(file, newFile);
} catch (FileAlreadyExistsException x) {
  // We ignore this
}
return CONTINUE;
```