Remove GUI's "Generate Coins" option from the menu.

CPU mining remains available via command line option [until/unless
the community decides to remove that completely].

db.cpp

@@ -778,10 +778,10 @@ bool CWalletDB::LoadWallet()
                 string strKey;
                 ssKey >> strKey;
 
-                // Menu state
-                if (strKey == "fGenerateBitcoins")  ssValue >> fGenerateBitcoins;
-
                 // Options
+#ifndef GUI
+                if (strKey == "fGenerateBitcoins")  ssValue >> fGenerateBitcoins;
+#endif
                 if (strKey == "nTransactionFee")    ssValue >> nTransactionFee;
                 if (strKey == "addrIncoming")       ssValue >> addrIncoming;
                 if (strKey == "fLimitProcessors")   ssValue >> fLimitProcessors;