Simplify poll select options helper

The `include_all` parameter was always used, and the option was
redundant because we already had a prompt offering the same
functionality.

I guess one possible reason was users would want to filter by all polls,
and having to click on "select a poll" to do so wasn't that intuitive.
So we're using "All" as the prompt instead.

app/helpers/polls_helper.rb

@@ -1,16 +1,11 @@
 module PollsHelper
-  def poll_select_options(include_all = nil)
+  def poll_select_options
     options = @polls.map do |poll|
       [poll.name, current_path_with_query_params(poll_id: poll.id)]
     end
-    options << all_polls if include_all
     options_for_select(options, request.fullpath)
   end
 
-  def all_polls
-    [I18n.t("polls.all"), admin_questions_path]
-  end
-
   def poll_dates(poll)
     if poll.starts_at.blank? || poll.ends_at.blank?
       I18n.t("polls.no_dates")

app/views/admin/poll/questions/_filter.html.erb

@@ -1,7 +1,7 @@
 <%= form_tag "", method: :get do %>
   <%= label_tag :poll_id, t("admin.questions.index.filter_poll") %>
   <%= select_tag "poll_id",
-                 poll_select_options(true),
-                 prompt: t("admin.questions.index.select_poll"),
+                 poll_select_options,
+                 prompt: t("polls.all"),
                  class: "js-location-changer" %>
 <% end %>