Bug 31141: Remove 'select_column' from waiting_holds.inc

Bug 31141: Remove 'select_column' from waiting_holds.inc

In the only place we use the waiting_holds include, we were hard coded
to always pass the 'select_column' variable as true.

We can simply and clarify the logic by just removing this superflous
variable.

Test plan
1. Confirm that the waiting holds tables continue to display as expected

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>