Remove a pseudo-bogus test for pthreads in the manager process.

The subsequent cleanup/separation of .h files makes this test
much less important, and it's not like it is a conclusive or
comprehensive test in the first place.

Fixes	#1230

bin/varnishd/mgt/mgt_main.c

@@ -655,7 +655,3 @@ main(int argc, char * const *argv)
 		(void)VPF_Remove(pfh);
 	exit(exit_status);
 }
-
-#if defined(PTHREAD_CANCELED) || defined(PTHREAD_MUTEX_DEFAULT)
-#error "Keep pthreads out of in manager process"
-#endif