Core: Drop workarounds for unsupported browsers

One test could be dropped. There is also a `detectDuplicates` one that claims is for Chrome before version 14 but even in current Chrome this test returns `false` (`true` in other browsers). What's going on here?

cc @gibson042
I'd like to see the `support.getElementsByClassName` assignment remain up with the other support checks so we have a single place to look.