transaction: Do not make Transaction to implement error interface anymore

As per previous commit, Transaction can't be used anymore as an error
value, but we instead we always return the status code.

transaction.go

@@ -126,8 +126,6 @@ func cbPAMConv(s C.int, msg *C.char, c C.uintptr_t) (*C.char, C.int) {
 }
 
 // Transaction is the application's handle for a PAM transaction.
-//
-//nolint:errname
 type Transaction struct {
 	handle     *C.pam_handle_t
 	conv       *C.struct_pam_conv
@@ -217,10 +215,6 @@ func start(service, user string, handler ConversationHandler, confDir string) (*
 	return t, nil
 }
 
-func (t *Transaction) Error() string {
-	return Error(t.lastStatus.Load()).Error()
-}
-
 // Item is a an PAM information type.
 type Item int