message: use html-to-text scraper for html parts

We were dumping the HTML-parts as-is in the Xapian indexer; however,
it's better to remove the html decoration first, and just pass the text.

We use the new built-in html->text scraper for that.

lib/message/mu-fields.cc

@@ -139,7 +139,6 @@ static void
 test_prefix()
 {
 	static_assert(field_from_id(Field::Id::Subject).xapian_prefix() == 'S');
-	static_assert(field_from_id(Field::Id::XBodyHtml).xapian_prefix() == 0);
 }
 
 [[maybe_unused]]