MAINTAINERS: move the BFQ io scheduler to orphan state

Nobody is maintaining this code, and it just falls under the umbrella
of block layer code. But at least mark it as such, in case anyone wants
to care more deeply about it and assume the responsibility of doing so.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>