Bug 19301: Move C4::Reserves::OnShelfHoldsAllowed to get_onshelfholds_policy

Bug 19301: Move C4::Reserves::OnShelfHoldsAllowed to get_onshelfholds_policy

Following the pattern introduced by bug 19300, we are going to move the
OnShelfHoldsAllowed logic to Koha::IssuingRules->get_onshelfholds_policy

Test plan:
Make sure the onshelfholds policy is correct when placing a hold

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>