Remove CSS_SYSTEM aggregate file

Drupal 8’s CSS aggregation strategy has been improved slightly by removing the CSS_SYSTEM constant (which was briefly renamed to CSS_AGGREGATE_SYSTEM in D8). The CSS from System module is combined with CSS from other modules, thus reducing the number of aggregation files from 6 to 4.

Ensuring that the System CSS is loaded before a module's CSS is now accomplished with a 'weight' of CSS_COMPONENT - 10.

Drupal 7:

$page['#attached']['css'][ drupal_get_path('module', 'system') . '/css/system.module.css'] = array('group' => CSS_SYSTEM, 'every_page' => TRUE);

Drupal 8:

// Adjust the weights to load these early.
$page['#attached']['css'][ drupal_get_path('module', 'system') . '/css/system.module.css'] = array('weight' => CSS_COMPONENT - 10, 'every_page' => TRUE);