Bug 19299: Replace C4::Reserves::GetReservesForBranch with Koha::Holds->waiting

Bug 19299: Replace C4::Reserves::GetReservesForBranch with Koha::Holds->waiting

GetReservesForBranch simply returns the waiting holds, for a specific
branch of not. This can easily be replaced with a call to
Koha::Holds->waiting

To avoid any regressions, I reuse the exact conditions (priority = 0),
but I do not think it is useful.

Test plan:
Make sure the holds information are correctly displayed on the waiting
holds screen.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>