Drop support for OpenCV 2

We currently support OpenCV 2,3 and 4.

OpenCV 2 is old (OpenCV 3 has been release 3.5 years ago!), and since v4 has been released, maybe we can remove the support for openCV 2

There is a big difference between openCV 2 and 3 API (in particular for machine learning algorithms), which means we basically have an OpenCV2 and an OpenCV3 version of each MachineLearningModel. Most of the tests using openCV also need two baselines (one for v2 and one for v3).

To remove support for openCV2 we need to remove the OTB_OPENCV_3 variable and remove all the code defined when this variable is set to 0. OTB_OPENCV_3 is true for v3 and v4, which means removing it will have no impact and compatibility with openCV 4. We also need to remove all additional baselines.

Note: OpenCV 2.4.1 is packaged on ubuntu 16.04, while OpenCV 3+ is packaged on ubuntu 18.04 and 19.04.

The question is, should we do this for OTB 7.1.0, or in the next major release (8.0) ?