Risposte:
Per chiunque cerchi (da drupal.org/node/2676164 ):
/**
* Implements hook_page_attachments_alter().
*/
function HOOK_page_attachments_alter(array &$attachments) {
foreach ($attachments['#attached']['html_head'] as $key => $attachment) {
if ($attachment[1] == 'system_meta_generator') {
unset($attachments['#attached']['html_head'][$key]);
}
}
}