Documentation: ublk: remove dead footnote

A previous commit removed the use of this footnote, delete it.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 3fdf2ec7 ("Documentation: ublk: Drop Stefan Hajnoczi's message footnote")
Signed-off-by: Jens Axboe <axboe@kernel.dk>