JobSystem: remove the DONT_SIGNAL flag

A side effect of this flag was that we had to always use notify_all
when starting a job, instead of notify_one -- in practice, we found
experimentally that more calls to notify_one is cheaper than fewer
calls to notify_all.

libs/utils/benchmark/benchmark_JobSystem.cpp

@@ -51,7 +51,7 @@ static void BM_JobSystemAsChildren4k(benchmark::State& state) {
         for (auto _ : state) {
             auto root = js.create(nullptr, &emptyJob);
             for (size_t i = 0; i < 4095; i++) {
-                js.run(js.create(root, &emptyJob), JobSystem::DONT_SIGNAL);
+                js.run(js.create(root, &emptyJob));
             }
             js.runAndWait(root);
         }