Clean some legacy files and move some build files (#23699)

* Clean the "tools" directory. The "tools" directory contains only two
files, move them.
* The "external_renderer.go" works like "cat" command to echo Stdin to
Stdout , to help testing.
* The `// gobuild: external_renderer` is incorrect, there should be no
space: `//gobuild: external_renderer`
* The `fmt.Print(os.Args[1])` is not a well-defined behavior, and it's
never used.
* The "watch.sh" is for "make watch", it's somewhat related to "build"
* After this PR, there is no "tools" directory, the project root
directory looks slightly simpler than before.
* Remove the legacy "contrib/autoboot.sh", there is no
"gogs_supervisord.sh"
* Remove the legacy "contrib/mysql.sql", it's never mentioned anywhere.
* Remove the legacy "contrib/pr/checkout.go", it has been broken for
long time, and it introduces unnecessary dependencies of the main code
base.

tools/external_renderer.go

@@ -1,23 +0,0 @@
-// Copyright 2017 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-// gobuild: external_renderer
-
-package main
-
-import (
-	"fmt"
-	"io"
-	"os"
-)
-
-func main() {
-	if len(os.Args) > 1 {
-		fmt.Print(os.Args[1])
-	} else {
-		_, err := io.Copy(os.Stdout, os.Stdin)
-		if err != nil {
-			fmt.Println(err)
-		}
-	}
-}

tools/watch.sh

@@ -1,8 +0,0 @@
-#!/bin/bash
-set -euo pipefail
-
-make watch-frontend &
-make watch-backend &
-
-trap 'kill $(jobs -p)' EXIT
-wait