Revert "Unset $wgUseXVO"

Testing impact on a current production issue, will un-revert if this wasn't the culprit.

This reverts commit 22145c96e5724aaf8f2c354cf4270f952a3f330e.

Change-Id: Idc976713cf1bbeeaa304eadcb3429f6fde7a91d3