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.

config/danbooru_default_config.rb

@@ -188,19 +188,6 @@ module Danbooru
       [server_host]
     end
 
-    # Names of other Danbooru servers.
-    def other_server_hosts
-      @other_server_hosts ||= all_server_hosts.reject {|x| x == server_host}
-    end
-
-    def remote_server_login
-      "danbooru"
-    end
-
-    def archive_server_login
-      "danbooru"
-    end
-
     # The method to use for storing image files.
     def storage_manager
       # Store files on the local filesystem.
@@ -474,10 +461,6 @@ module Danbooru
       %w[duplicate image_sample md5_mismatch resized upscaled downscaled]
     end
 
-    def shared_dir_path
-      "/var/www/danbooru2/shared"
-    end
-
     def stripe_secret_key
     end