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/controllers/prompts/tags_controller.rb

@@ -1,9 +0,0 @@
-class Prompts::TagsController < ApplicationController
-  def index
-    @tags = Current.account.tags.all.alphabetically
-
-    if stale? etag: @tags
-      render layout: false
-    end
-  end
-end

app/helpers/rich_text_helper.rb

@@ -7,10 +7,6 @@ module RichTextHelper
     content_tag "lexxy-prompt", "", trigger: "@", src: prompts_users_path, name: "mention"
   end
 
-  def tags_prompt
-    content_tag "lexxy-prompt", "", trigger: "#", src: prompts_tags_path, name: "tag"
-  end
-
   def cards_prompt
     content_tag "lexxy-prompt", "", trigger: "#", src: prompts_cards_path, name: "card", "insert-editable-text": true, "remote-filtering": true, "supports-space-in-searches": true
   end

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

app/views/prompts/tags/_tag.html.erb

@@ -1,8 +0,0 @@
-<lexxy-prompt-item search="<%= tag.title %>" sgid="<%= tag.attachable_sgid %>">
-  <template type="menu">
-    #<%= tag.title %>
-  </template>
-  <template type="editor">
-    #<%= tag.title %>
-  </template>
-</lexxy-prompt-item>

app/views/prompts/tags/index.html.erb

@@ -1 +0,0 @@
-<%= render partial: "prompts/tags/tag", collection: @tags %>