Revert "feat: D & D による上位タグ設定（#184） (#186)"

This reverts commit 5f306b04b8.

backend/config/routes.rb

@@ -1,11 +1,6 @@
 Rails.application.routes.draw do
   resources :nico_tags, path: 'tags/nico', only: [:index, :update]
 
-  scope 'tags/:parent_id/children', controller: :tag_children do
-    post ':child_id', action: :create
-    delete ':child_id', action: :destroy
-  end
-
   resources :tags do
     collection do
       get :autocomplete