handle deleting line item from selection list more carefully

handle deleting line item from selection list more carefully

Once a line item is attached to a purchase order, removing it
from a selection list should mean detaching it from the list,
not deleting it outright.

The open-ils.acq.lineitem.delete method can be removed
once the old jubgrid code is finally purged.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_2_0@19259 dcc99617-32d9-48b4-a31d-7c20da2025e4