BUGFIX: unify the response method for record deletions (os7212)
BUGFIX: unify the response method for record deletions (os7212)

sessionMessage is an old way of returning responses - the consistent way
of doing this is now X-Status header, which then gets distrubuted via JS
in the CMS frontend.

dhensby/silverstripe-cms,silverstripe/silverstripe-cms,silverstripe-labs/silverstripe-reports,dhensby/silverstripe-reports,dhensby/silverstripe-reports,silverstripe/silverstripe-siteconfig,silverstripe/silverstripe-cms,silverstripe-labs/silverstripe-reports,jonom/silverstripe-cms,dhensby/silverstripe-cms,dhensby/silverstripe-cms,helpfulrobot/i-lateral-silverstripe-siteconfig,jonom/silverstripe-cms