Fix Action Text sanitizer initialization

Using `initializer` in the engine may run the block before application
initializers. Instead, use `config.after_initialize` to ensure that
application initializers take effect properly.

Also, don't bother deleting the config value, since that pattern isn't
needed here (as it is in other railties like action_view/railtie.rb).

actiontext/lib/action_text/engine.rb

@@ -83,8 +83,8 @@ def to_trix_content_attachment_partial_path
       ActionText::Attachment.tag_name = app.config.action_text.attachment_tag_name
     end
 
-    initializer "action_text.sanitizer_vendor" do |app|
-      if klass = app.config.action_text.delete(:sanitizer_vendor)
+    config.after_initialize do |app|
+      if klass = app.config.action_text.sanitizer_vendor
         ActionText::ContentHelper.sanitizer = klass.safe_list_sanitizer.new
       end
     end