Use a button to destroy poll question answer images

As mentioned in commits 5311daadf and bb958daf0, using links combined
with JavaScript to generate POST (or, in this case, DELETE) requests to
the server has a few issues.

Note that the AJAX response stopped working after replacing the link
with a button. Not sure about the reason, but, since this is one of the
very few places where we use AJAX calls to delete content, the easiest
solution is to stop using AJAX and be consistent with what we do in the
rest of the admin section.

config/locales/en/responders.yml

@@ -34,6 +34,7 @@ en:
       destroy:
         budget_investment: "Investment project deleted successfully."
         topic: "Topic deleted successfully."
+        poll_question_answer_image: "Image deleted successfully."
         poll_question_answer_video: "Answer video deleted successfully."
         valuator_group: "Valuator group deleted successfully"
         vote: "Vote deleted successfully"

config/locales/es/responders.yml

@@ -34,6 +34,7 @@ es:
       destroy:
         budget_investment: "Proyecto de gasto eliminado."
         topic: "Tema eliminado."
+        poll_question_answer_image: "Imagen eliminada correctamente."
         poll_question_answer_video: "Vídeo de respuesta eliminado."
         valuator_group: "Grupo de evaluadores eliminado correctamente"
         vote: "Voto eliminado correctamente"