Remove unused arkose_labs_verify_api_url col from application_settings

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/360272.

Usage of arkose_labs_verify_api_url column of application_settings has been removed 2 years ago and was ignored in Ignore arkose_labs_verify_api_url column of app... (!156618 - merged) (milestone M – 17.2). This MR drops the column as described in https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-the-column-release-m1 (milestone M+1 – 17.3).

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