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/auth/auth.go

@@ -214,10 +214,8 @@ func DecodeResponse(response string) (user, domain, passwd string, err error) {
 
 	// Split identity into "user@domain", if possible.
 	user = identity
-	idsp := strings.SplitN(identity, "@", 2)
-	if len(idsp) >= 2 {
-		user = idsp[0]
-		domain = idsp[1]
+	if u, d, ok := strings.Cut(identity, "@"); ok {
+		user, domain = u, d
 	}
 
 	// Normalize the user and domain. This is so users can write the username