Rollup merge of #132368 - compiler-errors:do-not-iterator, r=tgross35

Remove `do_not_const_check` from `Iterator` methods

This attribute is not yet used, but keeping them around seems unnecessarily risky. I don't believe we should be constifying the `Iterator` trait until we've fully thought out how const closures are gonna work and have transitively consified all of its (implementation) dependencies.

cc `@rust-lang/project-const-traits`
r? libs