WikiDiscoverWikisPager: remove use of WikiDiscover class #555

Job	Run time
call-mediawiki-tests-workflow (REL1_43, 8.1, 81, true, true, false) / REL1_43 \| PHP 8.1	1m 24s
call-mediawiki-tests-workflow (master, 8.1, 81, false, false, true) / master \| PHP 8.1	1m 13s
notify-irc	9s
	2m 46s

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