Remove unused method on subscription controller

What does this MR do?

Small clean up of the SubscriptionsController#customer_portal_new_subscription_url method. From what I can tell, this method is no longer being used so we should just remove it.

It was originally introduced in 05b45235 and stopped being called after changes in 35f41147.

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

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Security

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