`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.
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