Bug 29984: Remove unused method Koha::Patrons->anonymise_issue_history

Bug 29984: Remove unused method Koha::Patrons->anonymise_issue_history

The method is no longer used, and replaced by
Koha::Old::Checkouts->anonymize.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Patrons.t
=> SUCCESS: Tests still pass
3. Run:
   $ git grep anonymise_issue_history
=> SUCCESS: The code doesn't mention it
4. 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>