Bug 17364: (follow-up) Do not check excluded fields

Bug 17364: (follow-up) Do not check excluded fields

When clicking on "Select all" excluded fields are selected.
Even if they are not saved into preference, this is disturbing.

Test plan :
1) Edit BorroweUnwantedFields system preference
2) Click "Select all"
3) See that branchcode is not selected

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>