Remove references to guide-conditions

std::condition was removed in 454882dcb7fdb03867d695a88335e2d2c8f7561a
but there are still links to the guide. Removing them.
auto merge of #12539 : gmjosack/rust/master, r=alexcrichton

std::condition was removed in 454882dcb7fdb03867d695a88335e2d2c8f7561a
but there are still links to the guide. Removing them.