Revert r87129 "(bug 21196) Article::getContributors() no longer fail on PostgreSQL...

Revert r87129 "(bug 21196) Article::getContributors() no longer fail on PostgreSQL" -- breaks stuff under MySQL like this:

SkinTemplate::makeTalkUrlDetails given invalid pagename User:

Backtrace:

#0 /var/www/trunk/includes/SkinTemplate.php(691): SkinTemplate->makeTalkUrlDetails('User:')
#1 /var/www/trunk/includes/SkinTemplate.php(495): SkinTemplate->buildPersonalUrls(Object(OutputPage))
#2 /var/www/trunk/includes/OutputPage.php(1906): SkinTemplate->outputPage(Object(OutputPage))
#3 /var/www/trunk/includes/Wiki.php(402): OutputPage->output()
#4 /var/www/trunk/index.php(146): MediaWiki->finalCleanup()
#5 {main}

Seen trivially by going to login page while not logged in; some user check is failing and ending up with an improperly initialized object.