Cleanup FindOpenCV.cmake and deprecate OpenCV_LIBRARIES in favor of OpenCV_LIBS

[traversaro]

FindOpenCV.cmake contains a lot of legacy logic that is not useful for any OpenCV version >= 3, and that takes a lot of time if find_package(OpenCV) is called in a system without OpenCV. This PR removes the legacy logic, and deprecated the FindOpenCV.cmake module YCM, as downstream users should be able to just use the regular OpenCVConfig.cmake installed directly by CMake. As a consequence of this, downstream users should switch from OpenCV_LIBRARIES to OpenCV_LIBS.

[Nicogene]

@pattacini we should adapt which repository uses OpenCV_LIBRARIES:

• robotology: https://github.com/search?q=org%3Arobotology%20OpenCV_LIBRARIES%20&type=code
• icub-tech-iit :https://github.com/search?q=org%3Aicub-tech-iit%20OpenCV_LIBRARIES&type=code (NONE)

[pattacini]

@pattacini we should adapt which repository uses OpenCV_LIBRARIES:

• robotology: https://github.com/search?q=org%3Arobotology%20OpenCV_LIBRARIES%20&type=code
• icub-tech-iit :https://github.com/search?q=org%3Aicub-tech-iit%20OpenCV_LIBRARIES&type=code (NONE)

Yes, definitely, but it's not for this distro.