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/models/poll.rb

@@ -40,7 +40,6 @@ class Poll < ApplicationRecord
   accepts_nested_attributes_for :questions, reject_if: :all_blank, allow_destroy: true
 
   scope :for, ->(element) { where(related: element) }
-  scope :public_polls, -> { where(related: nil) }
   scope :current,  -> { where("starts_at <= ? and ? <= ends_at", Date.current.beginning_of_day, Date.current.beginning_of_day) }
   scope :expired,  -> { where("ends_at < ?", Date.current.beginning_of_day) }
   scope :recounting, -> { where(ends_at: (Date.current.beginning_of_day - RECOUNT_DURATION)..Date.current.beginning_of_day) }