Remove complex logic from pictures controller

This logic also does not work any longer since the removal
of the screen-size-based pagination.

app/controllers/alchemy/admin/pictures_controller.rb

@@ -111,15 +111,15 @@ def destroy
       private
 
       def pictures_per_page_for_size(size)
+        return items_per_page unless in_overlay?
         case size
         when 'small'
-          per_page = in_overlay? ? 25 : (items_per_page * 2.9).floor
+          25
         when 'large'
-          per_page = in_overlay? ? 4 : (items_per_page / 1.7).floor + 1
+          4
         else
-          per_page = in_overlay? ? 9 : (items_per_page / 1.0).ceil + 4
+          9
         end
-        per_page
       end
 
       def in_overlay?