refactor(codex): remove stale affinity cleanup leftovers

Drop the last affinity-related executor artifacts so the PR stays focused on the minimal Codex continuity fix set: stable prompt cache identity, stable session_id, and the executor-only behavior that was validated to restore cache reads.

internal/runtime/executor/codex_continuity.go

@@ -14,8 +14,6 @@ import (
 	"github.com/tidwall/sjson"
 )
 
-const codexAuthAffinityMetadataKey = "auth_affinity_key"
-
 type codexContinuity struct {
 	Key    string
 	Source string