Bug 29874: Remove unused method C4::SIP::ILS::Item::fill_reserve

Bug 29874: Remove unused method C4::SIP::ILS::Item::fill_reserve

This patch removes the unused method.

Likely a leftover from ancient ages. AddIssue already fills the hold and
is called in the Checkout.pm library.

To test:
1. Run:
   $ git grep fill_reserve
=> FAIL: Notice it is only mentioned in the line that defines it
2. Apply this patch
3. Repeat 1
=> SUCCESS: Removed.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>