boost: remove useless threadGroup parameter from Discover

src/net.h

@@ -37,10 +37,6 @@
 class CScheduler;
 class CNode;
 
-namespace boost {
-    class thread_group;
-} // namespace boost
-
 /** Time between pings automatically sent out for latency probing and keepalive (in seconds). */
 static const int PING_INTERVAL = 2 * 60;
 /** Time after which to disconnect, after waiting for a ping response (or inactivity). */
@@ -441,7 +437,7 @@ private:
     friend struct CConnmanTest;
 };
 extern std::unique_ptr<CConnman> g_connman;
-void Discover(boost::thread_group& threadGroup);
+void Discover();
 void StartMapPort();
 void InterruptMapPort();
 void StopMapPort();