Use a closure instead of PathRouterPatternReplacer

Use a closure instead of PathRouterPatternReplacer

In PHP 5.3+ it's simpler to use closures rather than a replacer class to
pass parameters through to the preg_replace_callback() callback.

Remove without deprecation internal class PathRouterPatternReplacer,
unused in core and Gerrit-hosted extensions.

The new implementation is very similar, except that I renamed some
variables for clarity.

Also fixed an incorrect doc comment parameter type.

Change-Id: I4cd3c0162acdb02d51ab5b7f03b0a16e0a818d99