Drop legacy OpenCV from manifest

The only component referencing OpenCV as an optional dependency
is FFmpeg. But the respected configuration is never built anymore
and the codebase is so ancient that it was already dropped in O
and P last month.

Change-Id: I487119e3e4da53dd3c17d24df0d5b5495846309f
Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>