Move GenerateBitcoins() call out of net.cpp's StartNode()

The internal miner is closely bound to the wallet engine,
not the blockchain engine.

src/net.cpp

@@ -2047,9 +2047,6 @@ void StartNode(void* parg)
     // Dump network addresses
     if (!NewThread(ThreadDumpAddress, NULL))
         printf("Error; NewThread(ThreadDumpAddress) failed\n");
-
-    // Generate coins in the background
-    GenerateBitcoins(GetBoolArg("-gen", false), pwalletMain);
 }
 
 bool StopNode()