refactor: start on removing FF for subscription tiers (#7596)

## Summary

Refactor: remove FF for subscription tier, remove legacy code for non
subscription tier logic.
 
## Review Focus

Preexisting cloud functionality impact.

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## Screenshots (if applicable)

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┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7596-refactor-start-on-removing-FF-for-subscription-tiers-2cc6d73d3650816bac3aef893e4f37cd)
by [Unito](https://www.unito.io)

src/platform/cloud/subscription/composables/useSubscription.ts

@@ -16,9 +16,9 @@ import { useDialogService } from '@/services/dialogService'
 import type { components, operations } from '@/types/comfyRegistryTypes'
 import { useSubscriptionCancellationWatcher } from './useSubscriptionCancellationWatcher'
 
-type CloudSubscriptionCheckoutResponse = {
-  checkout_url: string
-}
+type CloudSubscriptionCheckoutResponse = NonNullable<
+  operations['createCloudSubscriptionCheckout']['responses']['201']['content']['application/json']
+>
 
 export type CloudSubscriptionStatusResponse = NonNullable<
   operations['GetCloudSubscriptionStatus']['responses']['200']['content']['application/json']