I believe to delete your credit card information from an expired azure free trial subscription, you will need to either reactivate or upgrade your subscription. But i would suggest you to raise a billing related request to ensure this. Billing support request via Azure portal is free for all azure customers and doesn't require a paid support plan. Please select 'Billing' under 'issue type' while creating the support request.
https://azure.microsoft.com/en-in/support/create-ticket
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