Remove unnecessary jsonpFunction from webpack config

This is done automatically on v5
See https://webpack.js.org/blog/2020-10-10-webpack-5-release/#automatic-unique-naming
[MAILPOET-3214]

webpack.config.js

@@ -34,7 +34,6 @@ const baseConfig = {
     path: path.join(__dirname, 'assets/dist/js'),
     filename: (PRODUCTION_ENV) ? '[name].[hash:8].js' : '[name].js',
     chunkFilename: (PRODUCTION_ENV) ? '[name].[hash:8].chunk.js' : '[name].chunk.js',
-    jsonpFunction: 'mailpoetJsonp'
   },
   resolve: {
     modules: [