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
OK; I moved it here to be consistent with the rest of similar cases, e.g. `support.qsa` is set inline. I'll move it back.