vtkStdString: use `std::string` in internal code

...Vtkm/Filters/Testing/Cxx/TestVTKMGradientAndVorticity.cxx

Charts/Core/Testing/Cxx/TestLinePlot.cxx

Charts/Core/Testing/Cxx/TestScatterPlot.cxx

Charts/Core/vtkCategoryLegend.cxx

Charts/Core/vtkChartBox.cxx

Charts/Core/vtkChartLegend.cxx

Charts/Core/vtkChartParallelCoordinates.cxx

Charts/Core/vtkChartXY.cxx

Charts/Core/vtkPlot.cxx

Charts/Core/vtkPlotArea.cxx

Charts/Core/vtkPlotBar.cxx

Charts/Core/vtkPlotBox.cxx

Charts/Core/vtkPlotHistogram2D.cxx

Charts/Core/vtkScatterPlotMatrix.cxx

Common/Color/Testing/Cxx/TestColorSeries.cxx

Common/Color/Testing/Cxx/TestNamedColors.cxx

Common/Color/vtkColorSeries.cxx

Common/Color/vtkNamedColors.cxx

Common/Core/Testing/Cxx/TestArrayUserTypes.cxx

Common/Core/Testing/Cxx/TestSortDataArray.cxx