index.php: Remove copyright line and use HTTPS url for mediawiki.org

None of the other entry points (or any file for that matter)
lists the main MediaWiki copyright holders. This is already in
README and SpecialVersion. Afaik there is no special value for
index.php to have this as well (plus, it has become outdated
and incomplete).

Change-Id: Ic3e1ab5b452002160563750fa1537dfbdd66e312