Bug 20411: Remove StaffDetailItemSelection system preference

Bug 20411: Remove StaffDetailItemSelection system preference

This patch removes the 'StaffDetailItemSelection' along with any usages of it. Now, the checkboxes on an item details page are always there.

To test:
1) Apply patch, restart_all, updatedatabase
2) In sys prefs, search for 'StaffDetailItemSelection', nothing should show up.
3) Visit an items details page, make sure there are checkboxes next each item that allow you to perform modification/deletion.

Signed-off-by: Eric Garcia <cubingguy714@gmail.com>
Signed-off-by: Heather Hernandez <heather_hernandez@nps.gov>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>