gems: remove whenever gem.

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

Capfile

@@ -10,7 +10,6 @@ install_plugin Capistrano::SCM::Git
 # Include tasks from other gems included in your Gemfile
 require 'capistrano/rbenv'
 require 'capistrano/rails'
-require 'whenever/capistrano'
 require 'capistrano3/unicorn'
 require 'capistrano/deploytags'
 require 'new_relic/recipes'

Gemfile

@@ -7,7 +7,6 @@ gem "pg"
 gem "delayed_job"
 gem "delayed_job_active_record"
 gem "simple_form"
-gem "whenever", :require => false
 gem "sanitize"
 gem 'ruby-vips'
 gem 'net-sftp'

Gemfile.lock

@@ -140,7 +140,6 @@ GEM
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
     childprocess (3.0.0)
-    chronic (0.10.2)
     clockwork (2.0.4)
       activesupport
       tzinfo
@@ -503,8 +502,6 @@ GEM
     websocket-driver (0.7.5)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
-    whenever (1.0.0)
-      chronic (>= 0.6.3)
     xpath (3.2.0)
       nokogiri (~> 1.8)
     yard (0.9.26)
@@ -598,7 +595,6 @@ DEPENDENCIES
   unicorn-worker-killer
   view_component
   webpacker (= 6.0.0.beta.6)
-  whenever
 
 BUNDLED WITH
    2.1.4

bin/whenever

@@ -1,29 +0,0 @@
-#!/usr/bin/env ruby
-# frozen_string_literal: true
-
-#
-# This file was generated by Bundler.
-#
-# The application 'whenever' is installed as part of a gem, and
-# this file is here to facilitate running it.
-#
-
-require "pathname"
-ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../Gemfile",
-  Pathname.new(__FILE__).realpath)
-
-bundle_binstub = File.expand_path("../bundle", __FILE__)
-
-if File.file?(bundle_binstub)
-  if File.read(bundle_binstub, 300) =~ /This file was generated by Bundler/
-    load(bundle_binstub)
-  else
-    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
-Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
-  end
-end
-
-require "rubygems"
-require "bundler/setup"
-
-load Gem.bin_path("whenever", "whenever")

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