Issue #12469: partial revert of 024827a9db64, freebsd6 thread initialization

 * Don't create a thread at startup anymore to initialize the pthread library:
   it changes the behaviour of many functions related to signal handling like
   sigwait()
 * Reenable test_sigtimedwait_poll() on FreeBSD 6
diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index fe9dde6..4b61e18 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -144,10 +144,7 @@
  * Initialization.
  */
 
-/* On FreeBSD6, pthread_kill() doesn't work on the main thread before
-   the creation of the first thread */
-#if defined(_HAVE_BSDI) \
-    || (defined(__FreeBSD__) && __FreeBSD_version < 700000)
+#if defined(_HAVE_BSDI)
 static
 void _noop(void)
 {