Simplify nosleep logic

Avoid redundant 'while' conditions. It is enough to
check them in the outer loop.

Quick tested for no regression 10K games at 4 threads
ELO: -1.32 +-3.9 (95%) LOS: 25.6%
Total: 10000 W: 1653 L: 1691 D: 6656

No functional change.

src/thread.cpp

@@ -373,7 +373,5 @@ void ThreadPool::start_thinking(const Position& pos, const LimitsType& limits,
           RootMoves.push_back(RootMove(m));
 
   main()->thinking = true;
-
-  for (Thread* th : *this)
-      th->notify_one();
+  main()->notify_one(); // Wake up main thread: 'thinking' must be already set
 }