Move gitea-vet to use go tool (#35878)

Add it as a [tool
dependency](https://go.dev/doc/modules/managing-dependencies#tools),
eliminating the need for `build.go`.

build.go

@@ -1,14 +0,0 @@
-// Copyright 2020 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-//go:build vendor
-
-package main
-
-// Libraries that are included to vendor utilities used during Makefile build.
-// These libraries will not be included in a normal compilation.
-
-import (
-	// for vet
-	_ "code.gitea.io/gitea-vet"
-)