Remove `rake backup:<resource_name>:*` subtasks

Follow up for #368707 (comment 1044070436):

IMO we should deprecate these subtasks. There are better supported alternatives https://docs.gitlab.com/ee/raketasks/backup_gitlab.html#excluding-specific-directories-from-the-backup and https://docs.gitlab.com/ee/raketasks/restore_gitlab.html#excluding-tasks-on-restore

These subtasks only work in a very specific set of circumstances and they leave behind residue files that may interfere with subsequent backups and restores.