Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

services/convert/secret.go

@@ -1,18 +0,0 @@
-// Copyright 2023 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package convert
-
-import (
-	secret_model "code.gitea.io/gitea/models/secret"
-	api "code.gitea.io/gitea/modules/structs"
-)
-
-// ToSecret converts Secret to API format
-func ToSecret(secret *secret_model.Secret) *api.Secret {
-	result := &api.Secret{
-		Name: secret.Name,
-	}
-
-	return result
-}