Bug 34645: always use discount, sort1, and sort2 from MarcFieldsToOrder

Bug 34645: always use discount, sort1, and sort2 from MarcFieldsToOrder

These values are stored at the order level, not per item. This patch simply sets the order values from the values retrieved from MarcFieldsToOrder even inf MarcItemFields to order is being used for other fields

To test:
Setup -- Set systempreferences below

MarcFieldsToOrder:
price: 949$g
quantity: 949$k
budget_code: 949$l
discount: 949$m
sort1: 949$n
sort2: 949$q

MarcItemFieldsToOrder:
homebranch: 949$a
holdingbranch: 949$b
itype: 949$y
nonpublic_note: 949$x
public_note: 949$z
loc: 949$c
ccode: 949$8
notforloan: 949$7
uri: 949$u
copyno: 949$t
price: 949$g
replacementprice: 949$v
itemcallnumber: 949$o
quantity: 949$k
budget_code: 949$l

Stage the attached bib-303.marcxml file
Add to basket from the staged file
Note that discount and sort1 and sort2 are not populated per the Marc
Apply patch, restart all
Stage and add to basket again
Confirm discount and sort1 and sort2 are populated correctly

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>