Drop OpenCV 2 support

Summary

Remove support for OpenCV 2

Rationale

See #1979 (closed)

Implementation Details

The OTB_OPENCV_3 variable has been removed, along with all OpenCV 2 specific code.

Additional notes

This is still WIP because some multi baseline might be removed.

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
• Optionally, run git diff develop... -U0 --no-color | clang-format-diff.py -p1 -i on latest changes and commit