Remove unused created_by scope from Proposal

The "created_by" scope in Proposal is no longer used anywhere in the code.
It was introduced in 77dd604 and its last usage was dropped in commit 64258baf97
("Refactor getting the public activity information").

app/models/proposal.rb

@@ -89,7 +89,6 @@ class Proposal < ApplicationRecord
   scope :draft,          -> { excluding(published) }
 
   scope :not_supported_by_user, ->(user) { where.not(id: user.find_voted_items(votable_type: "Proposal")) }
-  scope :created_by,            ->(author) { where(author: author) }
 
   def publish
     update!(published_at: Time.current)