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.

tests/sqlite.ini.tmpl

@@ -111,6 +111,6 @@ ENABLED = true
 [markup.html]
 ENABLED = true
 FILE_EXTENSIONS = .html
-RENDER_COMMAND = `go run tools/external_renderer.go`
+RENDER_COMMAND = `go run build/test-echo.go`
 IS_INPUT_FILE = false
 RENDER_CONTENT_MODE=sanitized