Remove dead Tag::Attachable code and tags prompt

The tags prompt was added for the Fizzy Do command system, but became
dead code when commands were removed in 89af9066. The commands_prompt
was cleaned up but tags_prompt was missed.

Remove the Tag::Attachable concern, Prompts::TagsController, its views
and route, and the tags_prompt helper. Tag still includes
ActionText::Attachable directly, which is needed by the data transfer
system.

app/models/tag.rb

@@ -1,5 +1,5 @@
 class Tag < ApplicationRecord
-  include Attachable, Filterable
+  include ActionText::Attachable, Filterable
 
   belongs_to :account, default: -> { Current.account }
   has_many :taggings, dependent: :destroy

app/models/tag/attachable.rb

@@ -1,11 +0,0 @@
-module Tag::Attachable
-  extend ActiveSupport::Concern
-
-  included do
-    include ActionText::Attachable
-
-    def attachable_plain_text_representation(...)
-      "##{title}"
-    end
-  end
-end