thread/Thread: Start() returns void

Since we switched to C++ exceptions, there is no code path which
returns false.

src/thread/Thread.hxx

@@ -89,7 +89,7 @@ public:
 #endif
 	}
 
-	bool Start(void (*f)(void *ctx), void *ctx);
+	void Start(void (*f)(void *ctx), void *ctx);
 	void Join();
 
 private: