Bug 15344: Remove one occurrence in circ/circulation.pl

Bug 15344: Remove one occurrence in circ/circulation.pl

In C4::Circulation::CanBookBeIssued, $borrower->{flags} is called and
should be populated by GetMemberDetails

It fixes the following error on checking out:
Can't use string ("1900156") as a HASH ref while "strict refs" in use at
/home/koha/src/C4/Circulation.pm line 813.

Note: Error occurs with patrons with staff permissions, not with 'normal' patrons.
Error is fixed with this patch.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>