package/opencv3: drop support for ffmpeg

package/opencv3: drop support for ffmpeg

opencv3's code is not compatible with newer versions of ffmpeg, and
opencv3 is no longer maintained, so we have no choice but to disable
its ffmpeg support.

Fixes:

  https://autobuild.buildroot.net/results/9ae3911583cccb6362f33cd82e5eaafb059fdc76/

It's not clear which ffmpeg version bump broken the build exactly, but
this issue is definitely present in 2025.02.x as the following
defconfig fails to build in a similar way on 2025.02.x:

 BR2_aarch64=y
 BR2_TOOLCHAIN_EXTERNAL=y
 BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
 BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_AARCH64_GLIBC_STABLE=y
 BR2_PACKAGE_FFMPEG_NONFREE=y
 # BR2_PACKAGE_FFMPEG_FFMPEG is not set
 # BR2_PACKAGE_FFMPEG_INDEVS is not set
 # BR2_PACKAGE_FFMPEG_OUTDEVS is not set
 BR2_PACKAGE_OPENCV3=y
 BR2_PACKAGE_OPENCV3_LIB_VIDEOIO=y
 BR2_PACKAGE_OPENCV3_BUILD_PERF_TESTS=y
 BR2_PACKAGE_OPENCV3_WITH_FFMPEG=y
 BR2_PACKAGE_OPENCV3_INSTALL_DATA=y

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>