MAINTAINERS: remove Alessandro

I'm definitely not up to date on the subject matter any more
and haven't been able to comment on the messages on the topic.

Signed-off-by: Alessandro Rubini <rubini@ipvvis.unipv.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>