rt: Remove some bogus pthread settings from rust_task_thread

This is all handled by rust_thread, and 'true' isn't even a valid
value to pass to pthread_attr_setdetachestate

src/rt/rust_task_thread.h

@@ -24,7 +24,6 @@ private:
     const int id;
 
 #ifndef __WIN32__
-    pthread_attr_t attr;
     static pthread_key_t task_key;
 #else
     static DWORD task_key;