Remove staging environment.

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

config/unicorn/staging.rb

@@ -1,47 +0,0 @@
-# Set your full path to application.
-app_path = "/var/www/danbooru2/current"
-
-# Set unicorn options
-worker_processes 2
-
-preload_app true
-timeout 180
-listen "/tmp/.unicorn.sock", :backlog => 512
-
-# Spawn unicorn master worker for user apps (group: apps)
-user 'danbooru', 'danbooru'
-
-# Fill path to your app
-working_directory app_path
-
-# Should be 'production' by default, otherwise use other env
-rails_env = ENV['RAILS_ENV'] || 'staging'
-
-# Log everything to one file
-stderr_path "log/unicorn.log"
-stdout_path "log/unicorn.log"
-
-# Set master PID location
-pid "#{app_path}/tmp/pids/unicorn.pid"
-
-before_fork do |server, worker|
-  old_pid = "#{server.config[:pid]}.oldbin"
-
-  if File.exist?(old_pid) && server.pid != old_pid
-    begin
-      Process.kill("QUIT", File.read(old_pid).to_i)
-    rescue Errno::ENOENT, Errno::ESRCH
-      # someone else did our job for us
-    end
-  end
-
-  if defined?(ActiveRecord::Base)
-    ActiveRecord::Base.connection.disconnect!
-  end
-end
-
-after_fork do |server, worker|
-  if defined?(ActiveRecord::Base)
-    ActiveRecord::Base.establish_connection
-  end
-end