Override and remove Classy and/or Drupal Core CSS

Waiting on two issues:

First we need the new way of overriding/removing libraries/assets etc:
#2451411: Add libraries-override to themes' *.info.yml →

Second I need to review whats been moved from core/system etc to Classy and nuke what we don't want (probably all of it will be nuked)
🌱 [Meta] Move module.theme.css files to Classy or Seven Postponed: needs info