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.

config/routes.rb

@@ -190,7 +190,6 @@ Rails.application.routes.draw do
 
   namespace :prompts do
     resources :cards
-    resources :tags
     resources :users
 
     resources :boards do