Fix buffer.UDP destination override (#2356)

transport/pipe/pipe.go

@@ -3,7 +3,6 @@ package pipe
 import (
 	"context"
 
-	"github.com/xtls/xray-core/common/buf"
 	"github.com/xtls/xray-core/common/signal"
 	"github.com/xtls/xray-core/common/signal/done"
 	"github.com/xtls/xray-core/features/policy"
@@ -26,12 +25,6 @@ func WithSizeLimit(limit int32) Option {
 	}
 }
 
-func OnTransmission(hook func(mb buf.MultiBuffer) buf.MultiBuffer) Option {
-	return func(option *pipeOption) {
-		option.onTransmission = hook
-	}
-}
-
 // DiscardOverflow returns an Option for Pipe to discard writes if full.
 func DiscardOverflow() Option {
 	return func(opt *pipeOption) {