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
Yes, but that kind of check has shown to be problematic. In jQuery, we use `typeof`. In Sizzle, we use `rnative`.