Do not require active_storage/engine

The activestorage adapter is optional and stores might
use another attachment adapter (ie. paperclip).

Since bundler already requires activestorage if the store
decided to use activestorage adapter (either via a direct
dependency or the transient rails gem) we do not need
to require it in our engine.

This removes a LoadError for stores/engines that do not
use activestorage.

core/lib/spree/core.rb

@@ -6,7 +6,6 @@
 require "active_job/railtie"
 require "active_model/railtie"
 require "active_record/railtie"
-require "active_storage/engine"
 require "sprockets/railtie"
 
 require 'active_support/deprecation'