Update media/src/core/core.sort.js

Second pass at this -- basic idea is still that there's no need to call remove when we're going to set immediately after.

Some of what I'd seen was that empty string should behave the same as undefined, but upon doing a little more testing, it looks like calling removeAttribute on a field where the attribute is undefined actually may be faster than setting it to an empty string:
http://jsperf.com/remove-attribute-vs-set-to-false/4