Remove unneeded HAML upgrade script

We updated a long time ago and I now copied those scripts to a Gist to
share with others:

* https://gist.github.com/mkllnk/bab2d474d686a2e3b88914129aee63e1

script/haml-up.rb

@@ -1,22 +0,0 @@
-#!/usr/bin/env ruby
-# frozen_string_literal: true
-
-# Upgrade HAML attribute syntax to prepare for HAML 6.
-#
-# HAML 6 stopped supporting nested hash attributes other than `data` and `aria`.
-# We used to be able to write:
-#
-#     %div{ ng: { class: "upper", bind: "model" } }
-#
-# This needs to be written in a flat structure now:
-#
-#     %div{ "ng-class" => "upper", "ng-bind" => "model" }
-#
-# This script rewrites HAML files automatically. It may be used like:
-#
-#     git ls-files '*.haml' | while read f; do ./haml-up.rb "$f"; done
-#
-require "haml_up"
-
-puts ARGV[0]
-HamlUp.new.upgrade_file(ARGV[0])