vtkStdString: remove unused inclusions and forward decls

Charts/Core/Testing/Cxx/TestDiagram.cxx

Charts/Core/Testing/Cxx/TestQtDiagram.cxx

Charts/Core/vtkAxisExtended.cxx

Charts/Core/vtkPlotBox.h

Charts/Core/vtkPlotGrid.h

Charts/Core/vtkPlotParallelCoordinates.h

Charts/Core/vtkPlotPoints.h

Common/Core/vtkInformationStringVectorKey.cxx

Common/DataModel/Testing/Cxx/TestCompositeDataSets.cxx

Domains/Chemistry/Testing/Cxx/TestPeriodicTable.cxx

Filters/ParallelImaging/vtkPPairwiseExtractHistogram2D.cxx

Filters/Points/vtkSPHKernel.h

Filters/Statistics/Testing/Cxx/TestKMeansStatistics.cxx

GUISupport/Qt/vtkQWidgetWidget.cxx

GUISupport/Qt/vtkQtAnnotationLayersModelAdapter.cxx

GUISupport/Qt/vtkQtTableModelAdapter.cxx

GUISupport/Qt/vtkQtTreeModelAdapter.cxx

GUISupport/QtSQL/Testing/Cxx/TestQtSQLDatabase.cxx

IO/ADIOS2/vtkADIOS2CoreImageReader.cxx

IO/Core/vtkJavaScriptDataWriter.h