std: remove a workaround for privacy limitations
`std:🧵:Thread` implements a non-exported `NewThread` trait to allow for internal-only use of `Thread::new`. Nowadays we have `pub(crate)`, which accomplishes the same thing but much more idiomatically.
Rustdoc handles this correctly (I checked and I didn't see `Thread::new` on the rustdoc entry for `Thread` with this change), and the stage1 `rustc` emits the correct error still (I'm assuming that the stage1 compiler uses my `libstd`?):
```
$ ./build/x86_64-apple-darwin/stage1/bin/rustc test.rs
error: method `new` is private
--> test.rs:4:18
|
4 | let thread = thread::Thread::new(None);
| ^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
```
d7a09d4e65