Drop the sprite files from chosen.css for isis

Use IcoMoon font for all the signs used in chosen.

Icomoon is almost always loaded in the back end, so we use it for styling the dropdown selects and therefore reduce by one the http requests, and also remove the redundant code for retina displays.

Tests
Apply the patch and navigate to different menus in the administrator area. All the drop downs should be exactly the same.

B/C
None