Bug 30023: Make MarkIssueReturned use ->anonymize

Bug 30023: Make MarkIssueReturned use ->anonymize

Koha::Old::Checkout->anonymize takes care of checking the syspref and
raises an exception if not set. So no we can now leverage on it, instead
of checking manually and then tweaking the checkout object manually as
well.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Circulation/MarkIssueReturned.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>