No need to install bundler since it is a default gem since Ruby 2.6

This commit removes "gem install bundler --conservative" command from `bin/setup` command
because bundler is installed as a default gem since Ruby 2.6.0.

* Ruby 2.6.0 Released
https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-released/
> Bundler is now installed as a default gem.

The current Rails main branch requires Ruby 3.1.0 that is higher than 2.6.0.
https://github.com/rails/rails/blob/8bac99ad7a403ef52a5c97e7afa73c7bbcc67110/rails.gemspec#L12

Author: yahonda

railties/lib/rails/generators/rails/app/templates/bin/setup.tt

@@ -13,7 +13,6 @@ FileUtils.chdir APP_ROOT do
   # Add necessary setup steps to this file.
 
   puts "== Installing dependencies =="
-  system! "gem install bundler --conservative"
   system("bundle check") || system!("bundle install")
 <% if using_node? -%>