Bug 19059: Move C4::Reserves::CancelReserve to Koha::Hold->cancel

Bug 19059: Move C4::Reserves::CancelReserve to Koha::Hold->cancel

This patch adds a new Koha::Hold->cancel method and replaces the calls
to C4::Reserves::CancelReserve with it.

Test plan:
- Add and cancel holds
- Change priority of holds

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>