Remove duplicate scope

The scopes `created_by_admin` and `public_polls` were very similar. I'm
using `created_by_admin` because `Poll.public_polls` feels redundant,
and the reason for that name is we should not name the scope `public`
because `public` is a ruby access modifier.

app/controllers/polls_controller.rb

@@ -13,7 +13,7 @@ class PollsController < ApplicationController
 
   def index
     @polls = Kaminari.paginate_array(
-      @polls.public_polls.not_budget.send(@current_filter).includes(:geozones).sort_for_list
+      @polls.created_by_admin.not_budget.send(@current_filter).includes(:geozones).sort_for_list
     ).page(params[:page])
   end