Remove Webpacker files in Action Text dummy app

These are leftover after Webpacker was replaced with importmaps in
89a24d6216eb771c1d9bdb7cef883ca50001ca93

actiontext/test/dummy/.browserslistrc

@@ -1 +0,0 @@
-defaults

actiontext/test/dummy/bin/webpack

@@ -1,19 +0,0 @@
-#!/usr/bin/env ruby
-
-ENV["RAILS_ENV"] ||= ENV["RACK_ENV"] || "development"
-ENV["NODE_ENV"]  ||= "development"
-
-require "pathname"
-ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
-
-require "rubygems"
-require "bundler/setup"
-
-require "webpacker"
-require "webpacker/webpack_runner"
-
-APP_ROOT = File.expand_path("..", __dir__)
-Dir.chdir(APP_ROOT) do
-  Webpacker::WebpackRunner.run(ARGV)
-end

actiontext/test/dummy/bin/webpack-dev-server

@@ -1,19 +0,0 @@
-#!/usr/bin/env ruby
-
-ENV["RAILS_ENV"] ||= ENV["RACK_ENV"] || "development"
-ENV["NODE_ENV"]  ||= "development"
-
-require "pathname"
-ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
-
-require "rubygems"
-require "bundler/setup"
-
-require "webpacker"
-require "webpacker/dev_server_runner"
-
-APP_ROOT = File.expand_path("..", __dir__)
-Dir.chdir(APP_ROOT) do
-  Webpacker::DevServerRunner.run(ARGV)
-end

actiontext/test/dummy/bin/yarn

@@ -1,11 +0,0 @@
-#!/usr/bin/env ruby
-APP_ROOT = File.expand_path('..', __dir__)
-Dir.chdir(APP_ROOT) do
-  begin
-    exec "yarnpkg", *ARGV
-  rescue Errno::ENOENT
-    $stderr.puts "Yarn executable was not detected in the system."
-    $stderr.puts "Download Yarn at https://yarnpkg.com/en/docs/install"
-    exit 1
-  end
-end