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.

config/locales/en/officing.yml

@@ -41,7 +41,6 @@ en:
     results:
       flash:
         create: "Results saved"
-        error_create: "Results NOT saved. Error in data."
         error_wrong_booth: "Wrong booth. Results NOT saved."
       new:
         title: "%{poll} - Add results"

config/locales/es/officing.yml

@@ -41,7 +41,6 @@ es:
     results:
       flash:
         create: "Datos guardados"
-        error_create: "Resultados NO añadidos. Error en los datos"
         error_wrong_booth: "Urna incorrecta. Resultados NO guardados."
       new:
         title: "%{poll} - Añadir resultados"