opencv: Remove hash file

Since the tarball we download is generated from GitHub, there's no
saying that the tarball won't differ in a month from now.
So the hash has no value there.

An update to the manual should be done.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>