Bug 19056: Replace C4::Reserves::GetReserveCount with Koha::Patron->holds->count

Bug 19056: Replace C4::Reserves::GetReserveCount with Koha::Patron->holds->count

This subroutine is only used once and can easily be replaced with
Koha::Patron->holds->count

Test plan:
- Set maxreserves=5
- Place 3 holds for a given patron
- Place again 3 holds for this patron
3+3 > 5 => The holds must not be placed

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>