rust private

Undo the Sized specialization from Iterator::nth I just added this as part of https://github.com/rust-lang/rust/pull/45595, but I'm now afraid there's a specialization issue with it, since I tried to add [another similar specialization](https://github.com/rust-lang/rust/compare/master...scottmcm:faster-iter-by-ref?expand=1#diff-1398f322bc563592215b583e9b0ba936R2390), and ended up getting really disturbing test failures like ``` thread 'iter::test_by_ref_folds' panicked at 'assertion failed: `(left == right)` left: `15`, right: `15`', src\libcore\../libcore/tests\iter.rs:1720:4 ``` So since this wasn't the most critical part of the change and a new beta is branching within a week, I think putting this part back to what it was before is the best option.