maintainers.inc: unassign Alistair Francis from opensbi

We were not able to get a response about availability over email, and so the recipe
has to be unassigned.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 60eda3dc)
Signed-off-by: Steve Sakoman <steve@sakoman.com>