remove 'const' from 'Option::iter'
This is kind of pointless to be a `const fn` since you can't do anything with the iterator. It is also the only `const fn iter*` in the entire standard library. It probably got constified when `~const` traits got added everywhere, and then was forgotten to be de-constified when that was undone.
The rest of the const_option feature seems like it can reasonably be stabilized, but this one IMO should not be stabilized, and it's not worth creating a new tracking issue.
Cc https://github.com/rust-lang/rust/issues/67441
38520aed18