... so if you really want to get rid of your draft directly, you can replace it by . and wait for the next autosave to kick in (notice the draft saved message below the edit box) ;-)

I must say I am most thankful that there is no Cancel button... it is one of those things you tend to press accidentally much more often than when you need it. Especially since Iain's answer below points out that you only need to start another answer somewhere else for your autosave to get deleted...