The `gallery` shortcode used to accept a SQL chunk for the value of the `orderby` attribute. The reason? `get_posts()` used to be called in the shortcode handler with a query-string blob of arguments passed to it. To mitigate breakage, `sanitize_sql_orderby()` was created in [7592].

`sanitize_sql_orderby()` expects a comma to be present when multiple `orderby` values were passed. The correct syntax for multiple fields is space-delimited. Since [29027], comma-separated values would never be parsed correctly when passed to `WP_Query->parse_orderby()`.

`sanitize_sql_orderby()` is used nowhere else in core, save for the `playlist` shortcode - I only added it there because I was mimic'ing the `gallery` logic. The function call can be removed from both shortcode handlers.
 
See #6476.
Fixes #23873.


git-svn-id: https://develop.svn.wordpress.org/trunk@30068 602fd350-edb4-49c9-b593-d223f7449a82