fix compile issue when DOASSERTS=y

Pthread pid caching was removed recently but an assert is still
present which checks pthread->pid, and this breaks the build when
debugging is enabled.

Reported-By: Bogdan Harjoc <harjoc@gmail.com>

libpthread/nptl/pthread_getattr_np.c

@@ -66,7 +66,6 @@ pthread_getattr_np (
     {
       /* No stack information available.  This must be for the initial
 	 thread.  Get the info in some magical way.  */
-      assert (abs (thread->pid) == thread->tid);
 
       /* Stack size limit.  */
       struct rlimit rl;