Move nTransactionFee from main.cpp to wallet.cpp

Transaction fee is only used by the wallet.
No need for it to be in main.cpp.

src/wallet.h

@@ -23,6 +23,9 @@
 #include <utility>
 #include <vector>
 
+// Settings
+extern int64_t nTransactionFee;
+
 class CAccountingEntry;
 class CCoinControl;
 class COutput;