Bug 29525: Make Koha::Hold->cancel anonymize if required

Bug 29525: Make Koha::Hold->cancel anonymize if required

This patch makes cancelling a hold anonymize it on the same call, if
settings require it (i.e. if borrowers.privacy is set to 2).

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Hold.t
=> SUCCESS: The code actually does what it is meant to
3. Try on the UI, cancelling a hold, noticing it gets anonymized if the
   patron has privacy == always/2.
4. Sign off :-D

Note: AnonymousPatron should be set. Otherwise it would set NULL. But
that's fine, that's what Koha does already.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>