Bug 9032: enforce consistent behavior when deleting lists

Bug 9032: enforce consistent behavior when deleting lists

DelShelf deletes a list regardless whether it is private, shared or
public. HandleDelBorrower had another approach, trying to save shared
and public lists by setting the owner to NULL.

This patch makes both routines behave consistently.

A new report 11889 has been opened to discuss the 'disowning' feature.

NOTE: I did not add a db revision here to handle possible cases of lists
without owner in the current data. Such public (or shared) lists can still
be used without any problem. Bug 11889 and a new planned report for a lists
management tool will address this topic further on. After that, all goals
of umbrella report 7310 should be realized.

Test plan:
Create a list P1 with user1 that allows adding by other users.
Add a patron (user2).
Login as user2 and create some lists, add some items.
Let user2 add some entries to P1 too.
Delete patron user2.
Verify that his lists are gone, but his entries in P1 are kept (nullified).

Signed-off-by: Dobrica Pavlinusic <dpavlin@rot13.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>