Remove no longer needed --keep-file-descriptors option

Bundler now keeps descriptors by default. While this behavior was
backported to Ruby 3.0.x, we're changing it now because, we've only
noticed it now that we're upgrading to Ruby 3.1.x, since it was first
developed for that version [1].

rubygems/rubygems@88b7a3e7e2

config/puma/defaults.rb

@@ -18,7 +18,6 @@
 workers 2
 preload_app!
 
-restart_command "bundle exec --keep-file-descriptors puma"
 plugin :tmp_restart
 
 on_restart do