gems: remove whenever gem.

We're now using the `clockwork` gem for cronjobs in production. See
config/initializers/clockwork.rb.

config/schedule.rb

@@ -1,26 +0,0 @@
-# This file is used by the `whenver` gem to generate a crontab that runs
-# Danbooru's maintenance tasks.
-#
-# @see app/logical/danbooru_maintenance.rb
-# @see https://github.com/javan/whenever
-
-# this is used in config/environments/production.rb.
-env "RAILS_LOG_TO_STDOUT", "true"
-
-set :output, "log/whenever.log"
-
-every :hour do
-  rake "maintenance:hourly"
-end
-
-every :day do
-  rake "maintenance:daily"
-end
-
-every :sunday do
-  rake "maintenance:weekly"
-end
-
-every :month do
-  rake "maintenance:monthly"
-end