How to stop featured image (post thumbnail) from having width and height in html

I am using a featured image (a.k.a. post thumbnail) in my posts, and unfortunately the HTML code automatically adds the width/height properties — I would prefer they be left blank as I am specifying size with CSS so the image scales to fit the viewport (I’ve created my own responsive theme). I can override this with width/height being set to “auto,” but is there a way to have the featured image show up without size being set in the HTML itself?

Thanks!