Remove unused UseVForm settings variables.

$wgUseVFormUserLogin and $wgUseVFormCreateAccount haven't done anything
for a while (only the new-style forms are present), so remove them and
$wmgUseVForm.

Bug: 46333
Change-Id: Icf3cbedf3508a9a5baf1732296b7db19b4d73d5c