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.
It'd be better, to only skip the config files that have the same name, not the whole directory. So for example, if currently es has a config dir that looks like this (for plugins `foo`):
```
/config/foo/bar.yml
```
and the new plugin that installs needs to copy two files there:
```
/config/foo/bar.yml
/config/foo/baz.yml
```
we'll skip `bar.yml` but we'll still copy `baz.yml`. This will help us a lot when we guide the user on how to upgrade - instead of telling the user, to copy & modify a bunch of files, we'll only need to tell him to modify files under `config/foo`