Bug 10504: Remove the unused C4::Acq::ModOrderItem routine

Bug 10504: Remove the unused C4::Acq::ModOrderItem routine

This routine has been introduced by commit 2d90fb22d43db986547bbaa945873d6dd78aab71.
The only call has been removed by commit 9eba7dc594fc0d4698da5e791d5f6f8682b2712a.

So now, this routine is useless.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 9f4f1ceafe218afcbc55d6dfb9ac1e32fc11803a)
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>