Remove legacy manifest provenance decoding

internal/manifest/manifest.go

@@ -1,7 +1,6 @@
 package manifest
 
 import (
-	"encoding/json"
 	"strings"
 	"time"
 
@@ -76,18 +75,6 @@ type EffectiveConfigProvenance struct {
 	EffectiveConfigDigest string                     `json:"effective_config_digest,omitempty"`
 }
 
-// UnmarshalJSON accepts unknown informational provenance from older manifests.
-// The enclosing manifest and its execution state remain strictly decoded.
-func (p *EffectiveConfigProvenance) UnmarshalJSON(data []byte) error {
-	type provenance EffectiveConfigProvenance
-	var decoded provenance
-	if err := json.Unmarshal(data, &decoded); err != nil {
-		return err
-	}
-	*p = EffectiveConfigProvenance(decoded)
-	return nil
-}
-
 // SelectedOverlayProvenance identifies an explicitly or default-selected
 // pipeline overlay without recording overlay content.
 type SelectedOverlayProvenance struct {