Bug 18256: Koha::Items - Remove GetItemsCount

Bug 18256: Koha::Items - Remove GetItemsCount

C4::Items::GetItemsCount can be replaced with Koha::Biblio->items->count

Test plan:
Create a bibliographic record with items attached
Try to delete the record from a basket (acquisition module), the detail
page and the batch item deletion tool.

=> You should not be able to delete it.

Remove the items and then try again to delete the record

=> Now you must be able to delete it.

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>