Don't require email templates presence for webpush notifications. Fixes

phpbb-extensions#91.

notification/method/webpush.php

@@ -97,7 +97,7 @@ public function get_type(): string
 	*/
 	public function is_available(type_interface $notification_type = null): bool
 	{
-		return parent::is_available($notification_type) && $this->config['wpn_webpush_enable']
+		return $this->config['wpn_webpush_enable']
 			&& !empty($this->config['wpn_webpush_vapid_public']) && !empty($this->config['wpn_webpush_vapid_private']);
 	}