storage manager: remove hierarchical option.

Remove the `hierarchical` file storage option. This means that image
files are always stored in MD5-based subdirectories, like this:

   https://danbooru.donmai.us/data/original/f3/a7/f3a70a89c350b5ed4db22dbb25b934bb.jpg
   https://danbooru.donmai.us/data/sample/f3/a7/sample-f3a70a89c350b5ed4db22dbb25b934bb.jpg
   https://danbooru.donmai.us/data/preview/f3/a7/f3a70a89c350b5ed4db22dbb25b934bb.jpg

instead of in a single flat directory, like this:

   https://danbooru.donmai.us/data/original/f3a70a89c350b5ed4db22dbb25b934bb.jpg

This option is removed because storing files in a single directory is a
bad idea for large installations, and migrating from a single directory
to subdirectories later is a pain.

Downstream boorus who still have files in the old layout can migrate by
running this script:

   `./script/fixes/077_symlink_subdirectories.rb`

This will create symlinks that redirect the 00-ff subdirectories back to
the current directory, so that you can still store files in a single
directory, but use URLs containing subdirectories.

You should also make sure to remove the `hierarchical` option from
`storage_manager` in `config/danbooru_local_config.rb` if you set it
there.

app/logical/storage_manager.rb

@@ -1,12 +1,11 @@
 class StorageManager
   class Error < StandardError; end
 
-  attr_reader :base_url, :base_dir, :hierarchical, :tagged_filenames
+  attr_reader :base_url, :base_dir, :tagged_filenames
 
-  def initialize(base_url:, base_dir:, hierarchical: false, tagged_filenames: Danbooru.config.enable_seo_post_urls)
+  def initialize(base_url:, base_dir:, tagged_filenames: Danbooru.config.enable_seo_post_urls)
     @base_url = base_url.chomp("/")
     @base_dir = base_dir
-    @hierarchical = hierarchical
     @tagged_filenames = tagged_filenames
   end
 
@@ -98,11 +97,7 @@ class StorageManager
   end
 
   def subdir_for(md5)
-    if hierarchical
-      "#{md5[0..1]}/#{md5[2..3]}/"
-    else
-      ""
-    end
+    "#{md5[0..1]}/#{md5[2..3]}/"
   end
 
   def seo_tags(post)