MAINTAINERS: remove linux@lists.openrisc.net

$ host -t mx lists.openrisc.net
Host lists.openrisc.net not found: 3(NXDOMAIN)

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>