strings: remove unnecessary `c_str()` calls in stream operators

Charts/Core/vtkPlot.cxx

Charts/Core/vtkPlotBag.cxx

Common/Color/Testing/Cxx/TestColorSeries.cxx

Common/Core/Testing/Cxx/TestArrayUniqueValueDetection.cxx

Common/Core/vtkGarbageCollector.cxx

Common/Core/vtkVariant.cxx

Common/DataModel/vtkDataAssembly.cxx

Common/ExecutionModel/vtkExecutive.cxx

Examples/ImageProcessing/Cxx/ImageBenchmark.cxx

Filters/Core/vtkArrayRename.cxx

Filters/Extraction/vtkExtractBlockUsingDataAssembly.cxx

Filters/General/vtkMultiThreshold.cxx

Filters/General/vtkSplitColumnComponents.cxx

Filters/Hybrid/vtkFacetReader.cxx

Filters/Parallel/vtkExtractCTHPart.cxx

...y/Testing/Cxx/TestPolyhedralMeshDistributedDataFilter.cxx

...Statistics/Testing/Cxx/TestRandomPMomentStatisticsMPI.cxx

Filters/Sources/vtkHyperTreeGridSource.cxx

Filters/Sources/vtkParametricFunctionSource.cxx

Filters/Statistics/vtkAutoCorrelativeStatistics.cxx