Remove unused url method in poll question answers

This method would never work because it relies on the `image`
association, instead of the `images` association defined in the
`Galleryable` module.

app/models/concerns/galleryable.rb

@@ -4,9 +4,5 @@ module Galleryable
   included do
     has_many :images, as: :imageable, inverse_of: :imageable, dependent: :destroy
     accepts_nested_attributes_for :images, allow_destroy: true, update_only: true
-
-    def image_url(style)
-      image&.attachment&.url(style) || ""
-    end
   end
 end