Remove unused by_author scope from Poll::PartialResult

The "by_author" scope in Poll::PartialResult is no longer used anywhere in
the code. Its usage was replaced by Poll::Answer.by_author in commit
6bc4f5b307 ("adds Poll::Answer model for web users").

app/models/poll/partial_result.rb

@@ -15,7 +15,6 @@ class Poll::PartialResult < ApplicationRecord
   validates :origin, inclusion: { in: ->(*) { VALID_ORIGINS }}
   validates :option, uniqueness: { scope: [:booth_assignment_id, :date] }, allow_nil: true
 
-  scope :by_author, ->(author_id) { where(author_id: author_id) }
   scope :by_question, ->(question_id) { where(question_id: question_id) }
 
   before_save :update_logs