package/opencv: drop package

opencv 2.x has seen no release since July 2018 and all buildroot
packages are already compatible with opencv3 so drop it to ease the
addition of opencv4 which is not fully backward compatible with opencv3
as "a lot of C API from OpenCV 1.x has been removed.":
https://opencv.org/opencv-4-0

.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>