Clean up ci_require_credit_card_on_trial_plan feature flag

What does this MR do and why?

Partially implements https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/763.

This MR cleans up ci_require_credit_card_on_trial_plan feature flag which has been in disabled state for 12 months and replaced with https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/32+.

A separate MR cleans up unused frontend code as a result of this removal.

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