Git: remove unused open3 require

I refactored the `Git` strategy to use `SystemCommand` instead of
`Open3#capture3` in #13387 but I forgot to remove `require "open3"`
at the time. `Git` doesn't use `open3` now, so this removes the
unused `require`.

Library/Homebrew/livecheck/strategy/git.rb

@@ -1,7 +1,6 @@
 # typed: strict
 # frozen_string_literal: true
 
-require "open3"
 require "system_command"
 
 module Homebrew