Replace clean-webpack-plugin with initial cleanup in Robofile

We need to share the same output path (assets/build/js) in more configs
in order to generate a common manifest.json file.
The cleanup plugin and also build-in output.clean setting do the cleanup on emit
and that causes that they delete files we want to keep.
Using different output paths solves the problem with cleanup but it causes that we generate a bunch of manifest.json files
instead of one.
As simplest workaround I chose cleaning the output directory before we trigger the Webpack build in the Robofile.
[MAILPOET-3214]

RoboFile.php

@@ -66,6 +66,7 @@ class RoboFile extends \Robo\Tasks {
     if (!is_dir('assets/dist/js')) {
       mkdir('assets/dist/js', 0777, true);
     }
+    $this->_exec('rm -rf ' . __DIR__ . '/assets/dist/js/*');
     $env = ($opts['env']) ?
       sprintf('./node_modules/.bin/cross-env NODE_ENV="%s"', $opts['env']) :
       null;