Remove obsolete workaround in Action Mailer

The buggy interaction between config.paths and config.autoload_paths
described in the comment was fixed in 7.1.2.

Author: fxn

actionmailer/lib/action_mailer/railtie.rb

@@ -71,16 +71,6 @@ class Railtie < Rails::Railtie # :nodoc:
     initializer "action_mailer.set_autoload_paths", before: :set_autoload_paths do |app|
       options = app.config.action_mailer
       app.config.paths["test/mailers/previews"].concat(options.preview_paths)
-
-      # Preview paths configuration needs a pass.
-      #
-      # config.paths is cached as soon as it is accessed. Therefore, mutating
-      # paths["test/mailers/previews"] does not guarantee config.autoload_paths
-      # is going to include them.
-      #
-      # If config.paths was accessed before, config.autoload_paths is going to
-      # have whatever paths["test/mailers/previews"] had when cached.
-      app.config.autoload_paths.concat(options.preview_paths)
     end
 
     initializer "action_mailer.compile_config_methods" do