Bug 24468: Remove unused C4::Reserves::_get_itype subroutine

Bug 24468: Remove unused C4::Reserves::_get_itype subroutine

Since bug 21206, C4::Reserves::_get_itype is not longer used and should
be removed.

  commit 31c29fd31f557306233e6a2936148a5bb10b89a1
  Bug 21206: Replace C4::Items::GetItem

UPDATE: In the meanwhile another occurrence was added to Reserves.t, but
easy to replace

Test plan:
  % git grep _get_itype
must not return any occurrences.

If one needs it, Koha::Item->effective_itemtype must be used instead.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>