sched: don't schedule() in thread::wake()

The thread that's doing the waking is obviously running, so no need to
reschedule, and it's awkward if done from an interrupt.

sched.cc

@@ -72,7 +72,6 @@ void thread::wake()
     if (!_on_runqueue) {
         _on_runqueue = true;
         runqueue.push_back(this);
-        schedule();
     }
 }