Remove Drop impl of mpsc Receiver and (Sync)Sender This change removes the empty `Drop` implementations for `mpsc::Receiver`, `mpsc::Sender` and `mpsc::SyncSender`. These implementations do not specify `#[may_dangle]`, so by removing them we make `mpsc` types play nice with drop check. This was previously attempted in [#105243](https://github.com/rust-lang/rust/pull/105243#issuecomment-1337188646) but then [abandoned due to a test failure](https://github.com/rust-lang/rust/pull/105243#issuecomment-1337227970). I've aligned the test with those for `Mutex` and `RwLock`.