Base: Drop QString-std::string conversion functions from Tools

Convenience helpers function Tools::toStdString and Tools::fromStdString
were implemented for Qt4 or older to perform utf8 aware conversion as
QString::toStdString/QString::fromStdString were using toAscii/fromAscii
internally (see https://dreamswork.github.io/qt4/classQString.html).

Since Qt5 QString uses toUtf8/fromUTf8, which makes the helper functions
obsolete (see https://doc.qt.io/qt-5/qstring.html#fromStdString).

src/Mod/Spreadsheet/App/Sheet.cpp

@@ -38,7 +38,6 @@
 #include <Base/FileInfo.h>
 #include <Base/Reader.h>
 #include <Base/Stream.h>
-#include <Base/Tools.h>
 
 #include "Sheet.h"
 #include "SheetObserver.h"
@@ -907,7 +906,7 @@ void Sheet::recomputeCell(CellAddress p)
     catch (const Base::Exception& e) {
         QString msg = QString::fromUtf8("ERR: %1").arg(QString::fromUtf8(e.what()));
 
-        setStringProperty(p, Base::Tools::toStdString(msg));
+        setStringProperty(p, msg.toStdString());
         if (cell) {
             cell->setException(e.what());
         }