docs: livecheck is now internal command

docs/External-Commands.md

@@ -37,16 +37,6 @@ For example, see the [header of `brew-services.rb`](https://github.com/Homebrew/
 
 ## Homebrew organisation external commands
 
-### homebrew-livecheck
-Check if there is a new upstream version of a formula.
-See the [`README`](https://github.com/Homebrew/homebrew-livecheck/blob/HEAD/README.md) for more info and usage.
-
-Install using:
-
-```sh
-brew tap homebrew/livecheck
-```
-
 ### homebrew-command-not-found
 Ubuntu's `command-not-found equivalent` for Homebrew.
 See the [`README`](https://github.com/Homebrew/homebrew-command-not-found/blob/HEAD/README.md) for more info and usage.