Bug 12841: aqorders fields should not be mapped

Bug 12841: aqorders fields should not be mapped

Bug 5336 introduced some code which should have been introduced by bug
7294.
Since the idea behind bug 7294 has been abandoned (map the aqorders
fields), the code can be removed.

Test plan:
Verify that Koha does not allow you to map the aqorders fields with a
MARC subfield.
Verify there is no regression on adding/updating an order.

Signed-off-by: Zeno Tajoli <z.tajoli@cineca.it>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>