(CE-1185) Fix fatal error on close account

When the new FacebookClient is enabled, the old FBConnect isn't available
and so removing the account from FBConnect fails. Add a check for the new
version and use its method of disconnecting accounts from Facebook.

This has stopped account closures from being logged since the start of the month, and potentially allows people with FB Connected accounts to still login after their account is closed.

Cherry-picked from dev (https://github.com/Wikia/app/pull/5830).

For approval by UConn before releasing. /cc @garthwebb @lizlux 
Thanks for fixing! Since you are at it, please make this line to be 
```php
} else if ( !empty( $wgEnableFacebookConnectExt ) ) {
```
And remove the conditional from https://github.com/Wikia/app/blob/release-255-CE-1185/extensions/wikia/EditAccount/SpecialEditAccount_body.php#L400, meaning, it will just call `disconnectFBConnect ` without conditions.