[Sunset] Remove the deprecated `created_by_description` and `created_by_title` fields

In [PR #162](https://github.com/fivetran/dbt_shopify/pull/162) the `created_by_description` and `created_by_title` fields were deprecated on `stg_shopify__discount_redeem_code` and `stg_shopify_gql__discount_redeem_code` (not referenced by any downstream model).

These fields have been marked as deprecated, but still need to be fully sunset (removed from the models, `get_*_columns` macros, yaml docs, and integration_tests seeds) in a future release.

Planned removal date: **October 1, 2026** (per GA-1033759).

Tracking here so the removal isn't lost — update this thread if the removal date changes.

Locate stg_shopify__discount_redeem_code and stg_shopify_gql__discount_redeem_code, then trace their get_*_columns macros, YAML documentation, and integration_tests seeds. Confirm the deprecated fields are not referenced downstream, remove them from each named location, and verify the package checks pass.

Written by the indexing model from the issue text.