remove easy-select2

this MR removes all the stuff that has been introduced for easy-select2 as we want to use django-select2 instead.

@mostafa.hadizadeh @bjoern.sass I already did this now as the django-app-template is using reuse and it would be a huge effort to figure out all the licenses for everything under mde_thredds/static/mde_thredds/vendor