Add a ctl command to remove the registration token for a target

[rustybee42]

Currently, if a mirrored target on a failed disk is replaced by a new one (filling it by resyncing), it is not possible to register it using the same target id as its generated token is different. It requires a manual database edit.

Add a command, e.g. target drop-registration-token that removes the token from the target so a new one can be inserted.

Update https://doc.beegfs.io/latest/trouble_shooting/general.html#why-does-the-management-reject-metadata-or-storage-target-registration