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
```

1 files changed, 1 insertions, 6 deletions

diff --git a/src/libstd/sys_common/thread_info.rs b/src/libstd/sys_common/thread_info.rs
index 95d8b6cc951..5ed48ee4558 100644
--- a/src/libstd/sys_common/thread_info.rs
+++ b/src/libstd/sys_common/thread_info.rs
@@ -31,7 +31,7 @@ impl ThreadInfo {
             if c.borrow().is_none() {
                 *c.borrow_mut() = Some(ThreadInfo {
                     stack_guard: None,
-                    thread: NewThread::new(None),
+                    thread: Thread::new(None),
                 })
             }
             Some(f(c.borrow_mut().as_mut().unwrap()))
@@ -54,8 +54,3 @@ pub fn set(stack_guard: Option<usize>, thread: Thread) {
         thread: thread,
     }));
 }
-
-// a hack to get around privacy restrictions; implemented by `std::thread`
-pub trait NewThread {
-    fn new(name: Option<String>) -> Self;
-}