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

core/core.go

@@ -9,8 +9,6 @@
 // connections.
 package core
 
-//go:generate go run github.com/xtls/xray-core/common/errors/errorgen
-
 import (
 	"fmt"
 	"runtime"