WikiDiscoverWikisPager: remove use of WikiDiscover class #551

Job	Run time
call-mediawiki-tests-workflow (master, 8.1, 81, false, false, true) / master \| PHP 8.1	2m 7s
call-mediawiki-tests-workflow (REL1_43, 8.1, 81, true, true, false) / REL1_43 \| PHP 8.1	2m 0s
notify-irc	26s
	4m 33s

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