Formally deprecate screen_icon() and get_screen_icon()

[DrewAPicture]

Previously, screen_icon() and get_screen_icon() were "informally" deprecated during the 3.8 cycle in [26537] for #26119.

The informal deprecation happened due to the proximity to final release. We're now more than 10 releases later and these two functions were never formally deprecated – that is, completed with _deprecated_function() calls.

This has now come up in #41121. So I think we should probably formally deprecate them now without much issue.

@Ippei Sumida Thanks for your patch! Would you mind updating it according to the inline docs recommendations in ​https://make.wordpress.org/core/handbook/best-practices/inline-documentation-standards/php/#1-2-deprecated-functions?

@swissspidy Thank you for your checking! inline docs is following now. It seems no problem, I think.
Would you mind telling me what's wrong?

<?php
/**
 * Displays a screen icon.
 *
 * @since 2.7.0
 * @since 3.8.0 Screen icons are no longer used in WordPress. This function no longer produces output.
 * @deprecated 3.8.0 Use get_screen_icon()
 * @see get_screen_icon()
 */

/**
 * Retrieves the screen icon (no longer used in 3.8+).
 *
 * @deprecated 3.8.0
 *
 * @return string
 */

Assigning to mark the good-first-bug as "claimed".