REFAC: remove GradientBoostedTree, as done in OpenCV 3

Documentation/SoftwareGuide/Latex/Classification.tex

...Applications/AppClassification/app/otbImageClassifier.cxx

...pplications/AppClassification/app/otbVectorClassifier.cxx

...ns/AppClassification/include/otbLearningApplicationBase.h

.../AppClassification/include/otbLearningApplicationBase.hxx

...AppClassification/include/otbTrainGradientBoostedTree.hxx

Modules/Applications/AppClassification/test/CMakeLists.txt

...s/Learning/LearningBase/include/otbMachineLearningModel.h

...ised/include/otbGradientBoostedTreeMachineLearningModel.h

...ed/include/otbGradientBoostedTreeMachineLearningModel.hxx

...clude/otbGradientBoostedTreeMachineLearningModelFactory.h

...ude/otbGradientBoostedTreeMachineLearningModelFactory.hxx

...ing/Supervised/include/otbMachineLearningModelFactory.hxx

...arning/Supervised/test/otbMachineLearningModelCanRead.cxx

...ing/Supervised/test/otbMachineLearningRegressionTests.cxx

Modules/Learning/Supervised/test/otbSupervisedTestDriver.cxx

...Learning/Supervised/test/otbTrainMachineLearningModel.cxx

Modules/Learning/Supervised/test/tests-opencv.cmake