Auto merge of #46074 - scottmcm:unspecialize-nth, r=bluss

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.

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