Remove legacy code (#3838)

* Add feature migration notice

* Remove legacy code of transport processing

* Clear legacy proto field

* Fix missing

* Unify protocolname

* Test remove

* Supressor

* Weird code

* Remove errorgen related comments

common/drain/drain.go

@@ -2,8 +2,6 @@ package drain
 
 import "io"
 
-//go:generate go run github.com/xtls/xray-core/common/errors/errorgen
-
 type Drainer interface {
 	AcknowledgeReceive(size int)
 	Drain(reader io.Reader) error