Bug 18405: Self checkout: Fix broken silent printing

Bug 18405: Self checkout: Fix broken silent printing

Fix broken 'silent printing' (without printer dialog) in self checkout.

To reproduce:
Set up silent printing to slip printers as described in:
https://wiki.koha-community.org/wiki/Setting_up_slip_printer_to_print_silently

Verify that it works for check-outs in staff client.
Verify that it does not work in Self checkout.

To test:
Apply patch
Verify that silent printing works on SCO

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>