Remove using namespace std from header file

It's considered bad form to import things into the global namespace in a
header. Put it in the cpp files where it is needed instead.

src/qt/transactiondesc.cpp

@@ -18,6 +18,8 @@
 #include <stdint.h>
 #include <string>
 
+using namespace std;
+
 QString TransactionDesc::FormatTxStatus(const CWalletTx& wtx)
 {
     AssertLockHeld(cs_main);