MINOR: Removed obsolete Subsite::$use_domain option (from r80859)
MINOR: Removed obsolete Subsite::$use_domain option (from r80859)

dnadesign/silverstripe-subsites,danaenz/silverstripe-subsites,silverstripe/silverstripe-subsites,petpack/silverstripe-subsites,danaenz/silverstripe-subsites,petpack/silverstripe-subsites,mandrew/silverstripe-subsites,dnadesign/silverstripe-subsites,mandrew/silverstripe-subsites,silverstripe/silverstripe-subsites,mikenz/silverstripe-simplesubsites,mikenz/silverstripe-simplesubsites,wernerkrauss/silverstripe-subsites,wernerkrauss/silverstripe-subsites