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.

test/controllers/prompts/tags_controller_test.rb

@@ -1,12 +0,0 @@
-require "test_helper"
-
-class Prompts::TagsControllerTest < ActionDispatch::IntegrationTest
-  setup do
-    sign_in_as :kevin
-  end
-
-  test "index" do
-    get prompts_tags_path
-    assert_response :success
-  end
-end