Remove enableBackBtn prop from core-modal
Remove enableBackBtn prop from core-modal

Fixes #2663

indirectlylit/kolibri,benjaoming/kolibri,jonboiser/kolibri,lyw07/kolibri,mrpau/kolibri,DXCanas/kolibri,indirectlylit/kolibri,learningequality/kolibri,learningequality/kolibri,mrpau/kolibri,christianmemije/kolibri,DXCanas/kolibri,learningequality/kolibri,mrpau/kolibri,DXCanas/kolibri,jonboiser/kolibri,jonboiser/kolibri,indirectlylit/kolibri,lyw07/kolibri,christianmemije/kolibri,jonboiser/kolibri,lyw07/kolibri,lyw07/kolibri,mrpau/kolibri,benjaoming/kolibri,benjaoming/kolibri,benjaoming/kolibri,learningequality/kolibri,indirectlylit/kolibri,christianmemije/kolibri,christianmemije/kolibri,DXCanas/kolibri