Remove unused go_back_to_new calls and unused error_create key

- Remove two redundant go_back_to_new calls in build_results, since
  @poll.questions.find already raises RecordNotFound if a question
  does not exist.
- Drop the fallback flash translation error_create, which is no longer
  used since commit 592fdffe4e and only remained as a default in
  go_back_to_new.
- Move check_officer_assignment from Officing::BaseController to
  Officing::ResultsController, its only place of use.

app/controllers/officing/base_controller.rb

@@ -13,12 +13,6 @@ class Officing::BaseController < ApplicationController
       raise CanCan::AccessDenied unless current_user&.poll_officer?
     end
 
-    def check_officer_assignment
-      if @officer_assignment.blank?
-        go_back_to_new(t("officing.results.flash.error_wrong_booth"))
-      end
-    end
-
     def load_officer_assignment
       @officer_assignments ||= current_user.poll_officer.officer_assignments.where(date: Time.current.to_date)
     end