#18 Add sql index for unpublished exercises

And remove spec for remove feature

db/migrate/20201208095929_add_index_to_rfc.rb

@@ -0,0 +1,6 @@
+class AddIndexToRfc < ActiveRecord::Migration[5.2]
+  def change
+    add_index(:request_for_comments, %i[user_id user_type created_at],
+              order: { user_id: :asc, user_type: :asc, created_at: :desc }, name: :index_rfc_on_user_and_created_at)
+  end
+end