Clean up rtk_payment_modal_migration feature flag

[talissoncosta]

Context

PR #7004 migrated Payment.js from Flux to modular functional components behind the rtk_payment_modal_migration feature flag. Once validated in production, we should remove the flag and the legacy code.

Tasks

• Remove rtk_payment_modal_migration feature flag check from BillingTab.tsx
• Delete legacy Payment.js
• Remove the legacy import from BillingTab.tsx
• Delete the feature flag from Flagsmith

Estimate

1 point