`tests/ui/issues/issue-29485.rs` has a useless `#[feature(recover)]`

[jieyouxu]

Remark (not for this PR): introduced in a17fb64, this #[feature(recover)] is (1) useless, as it's the outer attribute applied to the extern crate thus triggering unused_attributes but there's a #![allow(unused_attributes)] slapped in this test. And also #![feature(recover)] got stabilized in 1.9.

Originally posted by @jieyouxu in #157447 (comment)

NB: renamed to tests/ui/cross-crate/mut-ref-write-visible-after-unwind.rs in #157447.

Prob. (1) remove recover feature attr, (2) remove unused attr allow.

[jieyouxu]

Probably bundle into same PR w/ #157477

[lms0806]

@rustbot claim