REFAC: remove opencv2 specific code

.../Applications/AppClassification/include/otbTrainBoost.hxx

...ations/AppClassification/include/otbTrainDecisionTree.hxx

Modules/Applications/AppClassification/test/CMakeLists.txt

...earning/Supervised/include/otbBoostMachineLearningModel.h

...rning/Supervised/include/otbBoostMachineLearningModel.hxx

Modules/Learning/Supervised/include/otbCvRTreesWrapper.h

.../Supervised/include/otbDecisionTreeMachineLearningModel.h

...upervised/include/otbDecisionTreeMachineLearningModel.hxx

...rvised/include/otbKNearestNeighborsMachineLearningModel.h

...ised/include/otbKNearestNeighborsMachineLearningModel.hxx

...Supervised/include/otbNeuralNetworkMachineLearningModel.h

...pervised/include/otbNeuralNetworkMachineLearningModel.hxx

...g/Supervised/include/otbNormalBayesMachineLearningModel.h

...Supervised/include/otbNormalBayesMachineLearningModel.hxx

Modules/Learning/Supervised/include/otbOpenCVUtils.h

...Supervised/include/otbRandomForestsMachineLearningModel.h

...pervised/include/otbRandomForestsMachineLearningModel.hxx

.../Learning/Supervised/include/otbSVMMachineLearningModel.h

...earning/Supervised/include/otbSVMMachineLearningModel.hxx

Modules/Learning/Supervised/src/otbCvRTreesWrapper.cxx