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.

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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