Remove internal usage of require_dependency

Since 6983a89c726243fb38fe28cff49cba3a5f63ba6f `require_dependency` is
obsolete  since Rails stopped supporting classic mode.
It is only used by engines that need to support classic mode as well.

We can safely remove internal usage of `require_dependency`.

It was only used in the ActionMailer::Preview which is using just
`require` anyway.

actionmailer/lib/action_mailer/preview.rb

@@ -144,7 +144,7 @@ def preview_name
       private
         def load_previews
           preview_paths.each do |preview_path|
-            Dir["#{preview_path}/**/*_preview.rb"].sort.each { |file| require_dependency file }
+            Dir["#{preview_path}/**/*_preview.rb"].sort.each { |file| require file }
           end
         end