event/Thread: remove the Mutex

We don't need to access Thread::handle early inside the EventThread,
so we don't need this trick anymore.

src/event/Thread.cxx

@@ -26,7 +26,6 @@ EventThread::Start()
 {
 	assert(!thread.IsDefined());
 
-	const std::lock_guard<Mutex> protect(mutex);
 	thread.Start(ThreadFunc, this);
 }
 
@@ -44,11 +43,6 @@ EventThread::ThreadFunc()
 {
 	SetThreadName("io");
 
-	/* lock+unlock to synchronize with io_thread_start(), to be
-	   sure that io.thread is set */
-	mutex.lock();
-	mutex.unlock();
-
 	event_loop.Run();
 };