Clean up ci_require_credit_card_on_free_plan feature flag

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/762

This MR cleans up ci_require_credit_card_on_free_plan feature flag rolled out in https://gitlab.com/gitlab-org/gitlab/-/issues/330104 and disabled 11 months ago. The feature behind this flag has been replaced with https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/32+.

To keep the MR small, the other flag (ci_require_credit_card_on_trial_plan) will be removed in a separate MR as part of https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/763+.

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