Rollup merge of #40503 - swgillespie:thread-hack-removal, r=sfackler

std: remove a workaround for privacy limitations

`std::thread::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
```

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