Remove the screen_icon method from the admin class

[daniell]

The screen_icon method is deprecated, but still being used in xili-includes/xl-class-admin.php on line 1724.
It should be removed.

[dev-xiligroup]

Thanks for your note. Renewing XILI-LANGUAGE code is a long way and for some personal reasons, I have suspended rewriting for several months and hope to restart soon this about ten years sources.