Bug 18136: Fix ExportRemoveFields pre-fill behaviour

Bug 18136: Fix ExportRemoveFields pre-fill behaviour

Looks like it's caused by bug 13190.
Variable export_remove_fields is not sent to the template, let's
retrieve the syspref's value using the TT plugin.

Test plan:
Fill ExportRemoveFields with something (100a for instance)
Go on a checkout list page
At the bottom, the "do not export fields" input box should be pre-filled
with the content of the syspref

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>