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/protocol/protocol.go

@@ -1,3 +1 @@
 package protocol // import "github.com/xtls/xray-core/common/protocol"
-
-//go:generate go run github.com/xtls/xray-core/common/errors/errorgen