Bug 13740: Remove the NOT NULL clause in GetBorrowersToExpunge

Bug 13740: Remove the NOT NULL clause in GetBorrowersToExpunge

C4::Borrowers::GetBorrowersToExpunge should not use a "NOT IN", it is
not efficient at all.

With only 1 guarantor and more than 136k patrons, the not in clause in
this subroutine takes ages:
mysql> select count(*) FROM   borrowers where  borrowernumber NOT IN
(SELECT guarantorid FROM borrowers WHERE guarantorid IS NOT NULL AND
guarantorid <> 0) ;
[...]

not ended after 5min

With the query modified by this patch, the results come after 1 sec :)

Test plan:
Verify the delete_patrons.pl cronjob or the cleanborrowers tools work as
before.
Especially with guarantors.

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Koha Team AMU <koha.aixmarseille@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>