ffmpeg: remove dangling reference to BR2_arm10

BR2_arm10 has been removed in commit
d60489a6 ("arch: remove BR2_arm10t"),
so there is no point in keeping a reference to it in the ffmpeg
package.

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