CustomersDot: Remove code related to `payment_form_url` (`cc_validation`)
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### Problem
We have dead code related to the [`payment_form_url`](https://gitlab.com/gitlab-org/gitlab/-/blob/94243d5eb71f9f1f62011f544f7f87bd707b5403/ee/config/routes/directs/subscription_portal.rb#L11-13) in gitlab.com and [`cc_validation`](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/app/models/payment_form/new_credit_card.rb#L15) in customers.gitlab.com.
The use of this Zuora HPM was removed here: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167412/diffs#8671ae780923b8e431bb4fd401152fb4222b9e9c_49_0.
### Proposal
We should delete the code and Zuora HPM secrets in customers.gitlab.com:
1. Remove the PaymentForm: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/app/models/payment_form/new_credit_card.rb#L15
1. Remove the secrets (see how to [here](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/doc/security/rails_credentials.md#how-to-update-a-rails-credentials-vault)): https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/config/settings/application_secrets.yml#L75-76
1. Remove the documentation: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/doc/zuora/zuora_hpm_list.md#zuora-hpm-list
1. Update these tests:
1. https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/spec/requests/zuora_callbacks_request_spec.rb#L629-643
1. https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/spec/models/payment_form/new_credit_card_spec.rb#L14
1. [spec/requests/payment_forms_spec.rb](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/spec/requests/payment_forms_spec.rb#L26)
1. Remove this VCR filter: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/e1abfaa0c9f5da700a5dc1bd95f00229cbf82abb/spec/support/vcr.rb#L67-69
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issue