refactor(billing): drop the seat 410 stop-gap; PR 2 ships in the same release

The readable-copy branch only served the window where the seat-era plans
page was live without PR 2's replacement. PR 1 and PR 2 ship as one release,
so that window is zero: the typed 410 error, the classify branch, and their
tests go, leaving the PR with no seat-API changes at all (auto-provisioning
already fails soft on any create error).

Generated-By: PostHog Code
Task-Id: 1039ea23-9930-44e2-9888-b05cf8b129ec

Author: adboio