Prune git branches

[specious]

$ git branch -r
  origin/HEAD -> origin/production
  origin/add-airbrake
  origin/add-back-mobile
  origin/add-modal
  origin/add-simple-profanity-filter
  origin/add_docker
  origin/add_drain_types
  origin/additonal-edits-to-learn-more
  origin/allow-search-when-not-signed-in
  origin/appease-rubocop
  origin/fix-mailer-tests
  origin/hide-reminder-button
  origin/master
  origin/production
  origin/set_jquery_version
  origin/show-guidelines-when-logged-in
  origin/slight-edits-to-legend
  origin/support-renaming
  origin/update-feedback-email
  origin/update-first-email-copy
  origin/update-mailer-domains

I'm confused by all these branches. Perhaps we should delete the ones no longer relevant to our project?