Add API method to remove a suggestion (#7978)

DELETE /api/v1/suggestions/:account_id

When blocking, remove suggestion from both sides. Muting not affected,
since muting is supposed to be invisible to the target.

config/routes.rb

@@ -246,7 +246,7 @@ Rails.application.routes.draw do
 
       resources :streaming, only: [:index]
       resources :custom_emojis, only: [:index]
-      resources :suggestions, only: [:index]
+      resources :suggestions, only: [:index, :destroy]
 
       get '/search', to: 'search#index', as: :search