Remove check for append_assets_path initializer

It was removed from Rails in rails/rails@5172d93, and sprockets-rails
only supports Rails 5.2+

The initializer is also changed to be a simpler class method call
instead of reopening Rails::Engine, similarly to how it was changed in
rails/propshaft@c7d5542

Author: skipkayhil

lib/sprockets/railtie.rb

@@ -53,17 +53,6 @@ def precompiled_assets(clear_cache = false)
       @precompiled_assets ||= assets_manifest.find(config.assets.precompile).map(&:logical_path).to_set
     end
   end
-
-  class Engine < Railtie
-    # Skip defining append_assets_path on Rails <= 4.2
-    unless initializers.find { |init| init.name == :append_assets_path }
-      initializer :append_assets_path, :group => :all do |app|
-        app.config.assets.paths.unshift(*paths["vendor/assets"].existent_directories)
-        app.config.assets.paths.unshift(*paths["lib/assets"].existent_directories)
-        app.config.assets.paths.unshift(*paths["app/assets"].existent_directories)
-      end
-    end
-  end
 end
 
 module Sprockets
@@ -96,6 +85,12 @@ def configure(&block)
       end
     end
 
+    ::Rails::Engine.initializer :append_assets_path, :group => :all do |app|
+      app.config.assets.paths.unshift(*paths["vendor/assets"].existent_directories)
+      app.config.assets.paths.unshift(*paths["lib/assets"].existent_directories)
+      app.config.assets.paths.unshift(*paths["app/assets"].existent_directories)
+    end
+
     config.assets = OrderedOptions.new
     config.assets._blocks                    = []
     config.assets.paths                      = []