allow dragging of images in review screen again

when we were using webkit+file URLs, this deleted files from the
media folder! But we're behind a HTTP server now, so this should
be safe.

https://forums.ankiweb.net/t/no-longer-able-to-copy-latex-output/3128/5

originally disabled in
dd4bae6e8d

qt/ts/src/reviewer.ts

@@ -48,9 +48,6 @@ function _updateQA(html, fadeTime, onupdate, onshown) {
         }
         _runHook(onUpdateHook);
 
-        // don't allow drags of images, which cause them to be deleted
-        $("img").attr("draggable", "false");
-
         // render mathjax
         MathJax.Hub.Queue(["Typeset", MathJax.Hub]);