Remove $smwgCheckChangesBeforeUpdate

$smwgCheckChangesBeforeUpdate has be removed with

$ git log -SsmwgCheckChangesBeforeUpdate
commit 26504109157d79565668cf12a1ee89dff74fcf07
Author: Jeroen De Dauw <jeroendedauw@users.mediawiki.org>
Date:   Sat May 21 19:31:16 2011 +0000

    moving the change related stuff to SWL