Bug 31095: Remove keyed_on_code

Bug 31095: Remove keyed_on_code

We no longer require keyed_on_code from Koha::Patron::Restrictoin::Types
as we have relation accessors for the relevent use cases and no longer
reference the method anywhere in the codebase!

Test plan
1. Confirm 'keyed_on_code' is no longer referenced anywhere in the
   codebase.
   `git grep keyed_on_code`

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>