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.
you can reduce this code by using only the `nio` classes instead of moving forth and back between NIO and `File`, see `java.nio.files.Files` for things like moving `Path`