Remove stale PluginPurchaseController and purchase routes (#298)

The PluginPurchaseController and its routes were reintroduced by a stale
merge in PR #261 (cart-based checkout refactor), which was based on a
branch predating the tier-based pricing cleanup. The controller referenced
App\Services\GrandfatheringService which was intentionally deleted, causing
BindingResolutionException errors on any request to purchase routes.

The purchase flow was fully replaced by the cart system — nothing in the
app links to these routes. Remove the dead controller, views, and routes.

Closes #44

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: simonhamp