ApiFormatBase: Remove anonymous functions for PHP 5.2 compatibility

MediaWiki 1.19 is supposed to work with PHP 5.2; however, the fix
for bug 61362 ("HTML formatter for debug output from API allows HTML
injection") added anonymous functions, which are a feature introduced
in PHP 5.3.

Follows-up 7bcbd0c635ef, which was released as part of 1.19.13.

Bug: 63049
Co-Authored-By: Kevin Israel <pleasestand@live.com>
Change-Id: I1641646a61700741ab9b85d16c02fad66ab2cb65