Currently we cannot make 'srcset' image attribute effective even using 'source view editor' in our CKEditor.
By supporting extra plugin 'imageresponsive' we hope this could work.
Steps:
open Tour app and open any tour for editing, edit its body using below source, switch back and forth in 'source' view and 'srcset' attribute will disappear. Note that the data still saved into JCR and after saving we still able to use it in our front-end page.
Sample code:

<p>
  <img src="http://streaming1.danviet.vn/upload/1-2017/images/2017-03-27/14906333904788-unnamed--6-.jpg" 
  srcset="http://streaming1.danviet.vn/upload/1-2017/images/2017-03-27/14906333904788-unnamed--6-.jpg 1x, http://streaming1.danviet.vn/upload/1-2017/images/2017-03-27/14906333904788-unnamed--6-.jpg 2x" />
</p>