Bug 37809: (follow-up) Remove wrong embed

Bug 37809: (follow-up) Remove wrong embed

It seems to me like a copy and paste error coming from the holds
endpoint.

To test:
1. Use a REST API tool like Postman to ask for some patron's checkouts
2. Add x-koha-emebed: cancellation_requested
=> FAIL: It explodes, cannot embed it
3. Apply this patch
4. Repeat 2
=> SUCCESS: cancellation_requested no longer a valid option
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>