Remove unecessary unsafe, remove generated file in extra_snippets. (RustPython#4998)

Author: DimitrisJim

extra_tests/snippets/stdlib_os.py

@@ -32,6 +32,7 @@
     assert os.read(src_fd, src_len) == os.read(dest_fd, bytes_sent)
     os.close(src_fd)
     os.close(dest_fd)
+    os.remove('destination.md')
 
 try:
     os.open('DOES_NOT_EXIST', 0)

vm/src/stdlib/thread.rs

@@ -233,10 +233,8 @@ pub(crate) mod _thread {
             }
             let new_mut = RawRMutex::INIT;
 
-            unsafe {
-                let old_mutex: AtomicCell<&RawRMutex> = AtomicCell::new(&self.mu);
-                old_mutex.swap(&new_mut);
-            }
+            let old_mutex: AtomicCell<&RawRMutex> = AtomicCell::new(&self.mu);
+            old_mutex.swap(&new_mut);
 
             Ok(())
         }