Drop support for /cache/tag.json.

Drop support for https://danbooru.donmai.us/cache/tags.json. This was a
nightly dump of the tags table that was originally added in #1012. It
was never documented and never really used except for by the DanbooruUp
extension.

test/unit/danbooru_maintenance_test.rb

@@ -3,19 +3,9 @@ require 'test_helper'
 class DanbooruMaintenanceTest < ActiveSupport::TestCase
   context "daily maintenance" do
     setup do
-      # have ApiCacheGenerator save files to a temp dir.
-      @temp_shared_dir_path = "/tmp/#{SecureRandom.uuid}"
-      Danbooru.config.stubs(:shared_dir_path).returns(@temp_shared_dir_path)
-
-      FactoryBot.create(:tag, post_count: 1) # for ApiCacheGenerator
       FactoryBot.create(:admin_user) # for SuperVoter.init!
     end
 
-    teardown do
-      FileUtils.rm_rf(@temp_shared_dir_path)
-      Danbooru.config.unstub(:shared_dir_path)
-    end
-
     should "work" do
       assert_nothing_raised { DanbooruMaintenance.daily }
     end