Apply more mechanical simplifications using recent Go features

This is a follow-up on the `go fix` run in commit
0210c5fd92.

That commit recorded the automated updates.

This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.

It also removes some now-obsolete comments and statements.

internal/envelope/envelope.go

@@ -11,12 +11,8 @@ import (
 
 // Split an user@domain address into user and domain.
 func Split(addr string) (string, string) {
-	ps := strings.SplitN(addr, "@", 2)
-	if len(ps) != 2 {
-		return addr, ""
-	}
-
-	return ps[0], ps[1]
+	user, domain, _ := strings.Cut(addr, "@")
+	return user, domain
 }
 
 // UserOf user@domain returns user.
@@ -48,7 +44,7 @@ func AddHeader(data []byte, k, v string) []byte {
 		if v[len(v)-1] == '\n' {
 			v = v[:len(v)-1]
 		}
-		v = strings.Replace(v, "\n", "\n\t", -1)
+		v = strings.ReplaceAll(v, "\n", "\n\t")
 	}
 
 	header := []byte(fmt.Sprintf("%s: %s\n", k, v))