Remove Feature Flag: use_api_for_payment_validation

What does this MR do and why?

Describe in detail what your merge request does and why.

With this MR we are removing the following feature flag: use_api_for_payment_validation.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

This requires both gdk and CustomersDot running.

• Setup the HPM in Zuora Api Sandbox, you have two ways:
  1. create an HPM similar to this: https://apisandbox.zuora.com/apps/HostedPageLite.do?method=preview&id=8ad099157fd495bf017fdadbab7e6959 – for URL/host, use your gdk host
  2. just use mine 😇 (but please, do not change the configuration) – it assumes you run gdk on http://localhost:3000
• copy the id of the page (mine is 8ad099157fd495bf017fdadbab7e6959)
• go to secrets.yml in CustomersDot and use it for zuora_payment_method_validation_page_id in development settings (if not there, add it manually)
• (restart CustomersDot)
• in your terminal, export GITLAB_SIMULATE_SAAS=true (if gdk is already running, do a gdk restart)
• go to the gdk rails console and type Feature.enable(:ci_require_credit_card_on_free_plan)

How to see the iFrame

• Create a new User on your gdk. You can add it by visiting /admin/users/ and logging in with your root. The user should not be ad admin
• Log in with the new user
• Create a group and a project
• In the repository of the project, create a .gitlab-ci.yml file (you can select whatever template)
• Visit /-/pipelines/new under your project
• Click on Run pipeline
• An alert should appear (see screenshot), click on Validate account

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #360494 (closed)