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/thread/mod.rs b/src/libstd/thread/mod.rs
index 64c2be25222..edf928d6106 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -745,7 +745,7 @@ pub struct Thread {
 
 impl Thread {
     // Used only internally to construct a thread object without spawning
-    fn new(name: Option<String>) -> Thread {
+    pub(crate) fn new(name: Option<String>) -> Thread {
         let cname = name.map(|n| {
             CString::new(n).expect("thread name may not contain interior null bytes")
         });
@@ -858,11 +858,6 @@ impl fmt::Debug for Thread {
     }
 }
 
-// a hack to get around privacy restrictions
-impl thread_info::NewThread for Thread {
-    fn new(name: Option<String>) -> Thread { Thread::new(name) }
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // JoinHandle
 ////////////////////////////////////////////////////////////////////////////////