Unset $wgUseXVO

Remove the entry for wgUseXVO in InitialiseSettings.php, which is tantamount to
setting it to 'false' (the default).

Rationale:
- The header adds ~340 bytes on most Wikimedia wikis.
- Our current text caching setup doesn't handle XVO.
- XVO patch was not merged into upstream Squid, and is thus not widely
  implemented.

Change-Id: I5e0709ef0288f337f9476dedfabb7bd013ec3675