StringUtils: Deprecate Replacer classes

StringUtils: Deprecate Replacer classes

The Replacer classes were added in 1.9, when MediaWiki supported PHP 5.0
and 5.1. They were designed to be used with preg_replace_callback() and
StringUtils::delimiterReplaceCallback(). Now that Closures exist in PHP
5.3 and newer, there is no need to define a class for this purpose.

All existing Replacer subclasses are simple enough that their few uses
can easily be replaced with Closures, without making the code harder to
understand. In fact, the code probably becomes easier to understand, as
what each match is replaced with becomes more obvious -- no need to
refer to a separate class.

MediaWiki code search finds no uses in extensions. Thus, these classes
are hard deprecated immediately.

Change-Id: I441c21689909fb06a1ea07a305259eeb82cb2345