Remove _drupal_add_css() from common.inc

Meta issue: #2205673: [meta] Remove all @deprecated functions marked "remove before 8.0"

Only the function exists, it's usage is removed

[ashish@xeon:/var/www/html/d8/core/includes]$grep _drupal_add_css common.inc 
 * Calling drupal_static_reset('_drupal_add_css') will clear all cascading
 * _drupal_add_css() in a hook_page_build() implementation.
 *       being the same, a stylesheet added by a call to _drupal_add_css() that
 *       which _drupal_add_css() happened earlier in the page request.
function _drupal_add_css($data = NULL, $options = NULL) {
 * @see _drupal_add_css()
    $css = _drupal_add_css();
 *   of member items from _drupal_add_css() or _drupal_add_js().
 * @see _drupal_add_css()
      $css = _drupal_add_css();
 * @see _drupal_add_css()
  // The parameters for _drupal_add_js() and _drupal_add_css() require special
[ashish@xeon:/var/www/html/d8/core/includes]$