package/gstreamer1/gst1-plugins-bad: disable opencv plugin with opencv-3

The opencv plugin uses stuff from the now removed opencv's legacy module.
Note that the configure script already correctly check the opencv version
and disable the opencv plugin with opencv-3.x.

Also remove 'select ...' to prevent kconfig from complaining about
recursive dependencies.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>