Remove staging environment.

We never used any of this stuff. The only environments used are
production, development, and test.

config/deploy/staging.rb

@@ -1,6 +0,0 @@
-set :user, "danbooru"
-set :rails_env, "staging"
-server "testbooru.donmai.us", :roles => %w(web app db), :primary => true, :user => "danbooru"
-
-set :linked_files, fetch(:linked_files, []).push(".env.staging")
-set :deploy_to, "/var/www/danbooru2"