Merge pull request #26579 from y-yagi/remove_unnecessary_rails_env_setting

remove unnecessary `RAILS_ENV` setting

railties/lib/rails/generators/rails/app/templates/test/test_helper.rb

-ENV['RAILS_ENV'] ||= 'test'
 require File.expand_path('../../config/environment', __FILE__)
 require 'rails/test_help'
 

railties/lib/rails/generators/rails/plugin/templates/test/test_helper.rb

-# Configure Rails Environment
-ENV["RAILS_ENV"] = "test"
-
 require File.expand_path("../../<%= options[:dummy_path] -%>/config/environment.rb", __FILE__)
 <% unless options[:skip_active_record] -%>
 ActiveRecord::Migrator.migrations_paths = [File.expand_path("../../<%= options[:dummy_path] -%>/db/migrate", __FILE__)]