Simple Fees Phase 7: Remove Legacy Fees

Phase 7 of Simple Fees. Remove the old usage-based fee system now that simple fees is the default (`fees.simpleFeesEnabled` defaults to `true`).

PR #25473 already lands the handler and dispatcher removal. Remaining work is split into the sub-issues below: shared-class teardown, the high-volume gate and flag removal, fee-schedule data and tooling, and the per-service test-clients cleanup.

See #21642 for the original ordering notes.