(1/4) Revert "Correct image borders and principal point computation in cv::stereoRectify"

This reverts commit 93ff1fb2.

(2/4) Revert "fix calib3d changes in 6836 plus some others"

This reverts commit fa42a1cf.

(3/4) Revert "fix compiler warning"

This reverts commit b3d55489.

(4/4) Revert "add test for 6836"

This reverts commit d06b8c4e.