graph/db: remove unused sig field from ChannelEdgePolicy

Remove the cached parsed signature field and its lazy getter method
from ChannelEdgePolicy. This field was unused throughout the codebase
and the signature is already stored as raw bytes in SigBytes.

The SetSigBytes method is updated to remove the cache invalidation
logic.

routing/localchans/manager.go

@@ -449,7 +449,7 @@ func (r *Manager) updateEdge(chanPoint wire.OutPoint,
 	}
 
 	// Clear signature to help prevent usage of the previous signature.
-	edge.SetSigBytes(nil)
+	edge.SigBytes = nil
 
 	return nil
 }