refactor: drop dead affinity package and the metrics it owned

The affinity package (picker, store, reaper, ~310 lines of test) was
fully implemented but had zero callers — main.go didn't wire it,
config/rbac/role.yaml didn't grant ConfigMap permissions, and the
package doc on main.go was the only thing claiming the webhook 'owns
the per-pool node-affinity reservation store.' Cocoon-specs flagged it
as dead code in the recent /code review.

Delete the package, drop the now-unused affinity_reservations_total /
affinity_releases_total prometheus counters and their RecordReservation
/ RecordRelease helpers, drop the unused 'pool' label constant, and
fix the package doc on main.go. go.mod / go.sum tidy: github.com/
google/go-cmp and pmezard/go-difflib were pulled in transitively by
the affinity tests and are no longer referenced.

If we ever wire affinity reservations again, it'll be on a fresh design
(probably with proper RBAC up-front and a chosen storage layer); the
deleted code optimised for ConfigMap-as-store which is not necessarily
what we want.

Author: CMGS