Remove deprecated config.active_storage.replace_on_assign_to_many

`config.active_storage.replace_on_assign_to_many` has been removed and
this piece of code has become unreachable.

This is a follow-up to b06252e.

activestorage/lib/active_storage/engine.rb

@@ -123,10 +123,6 @@ class Engine < Rails::Engine # :nodoc:
           ActiveStorage.silence_invalid_content_types_warning = app.config.active_storage.silence_invalid_content_types_warning
         end
 
-        unless app.config.active_storage.replace_on_assign_to_many.nil?
-          ActiveStorage.replace_on_assign_to_many = app.config.active_storage.replace_on_assign_to_many
-        end
-
         ActiveStorage.track_variants = app.config.active_storage.track_variants || false
       end
     end