Fix: remove unnecessary arrow function

[ZER0]

This arrow function makes Uglify to fails in our project, plus I guess
it wouldn't work in browser that does not support them (as IE11).
Since it seems the only one in the project, and since it's also
unnecessary, I can see only benefits from remove it.

[sandstrom]

thanks @ZER0! 🏆

I think this will be undefined in the nested invocation, which won't cause any issues running the current code, but may cause hiccups if the code is changed further down the road.

How about something like this?

var _this = this;
Array.from(el.querySelectorAll('img')).every(function(img) {
  _this.isImageLoaded(img);
});

Edit: Never mind, can't see why we'd ever make that method recursive in the future. Let's go with this!