Don't autoload Vector.php using composer.json

As per T467, the RfC for extension management with composer has been
declined. Hence, we can get rid of Vector.php from composer.json, thus
also solving the issue of composer test die()ing because it loads
Vector.php and wfLoadSkin is not found.

Any users who are currently installing the skin via composer will need
to add "wfLoadSkin( 'Vector' )" to their LocalSettings.php.

Bug: T103800
Change-Id: Icd8acc34f52be9c3c0f564744d2eb2aae7f34398