API: Remove obsolete wgRightsCode from siteinfo

This was apparently a companion to $wgRightsPage, $wgRightsUrl,
$wgRightsText, and $wgRightsIcon, added in r3148.

But even then it was commented out as "not yet used", and appears to
have eventually disappeared without ever having been used.

Change-Id: I63f4393c22912f04e4ea690ebb1cd02bae65faab
Merge "API: Remove obsolete wgRightsCode from siteinfo"