metrics: Don't show "not mining" text for mainnet

Mining with the zcashd built-in CPU miner is not useful work on mainnet
at the current network difficulty.

src/metrics.cpp

@@ -373,7 +373,7 @@ int printMiningStatus(bool mining)
             }
         }
         lines++;
-    } else {
+    } else if (Params().NetworkIDString() != "main") {
         std::cout << _("You are currently not mining.") << std::endl;
         std::cout << _("To enable mining, add 'gen=1' to your zcash.conf and restart.") << std::endl;
         lines += 2;