Replace uses of deprecated MediaWiki\Message\Converter
The converter is no longer needed now that Message and MessageValue
use the same internal format for the message parameters.
Bug: T358779
Depends-On: I625a48a6ecd3fad5c2ed76b23343a0fef91e1b83
Change-Id: I41392aca4ae6b40f3476397d7ca37ba6cadb2ae4