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: 3fdf2ec ("Documentation: ublk: Drop Stefan Hajnoczi's message footnote")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

Documentation/block/ublk.rst

@@ -349,5 +349,3 @@ References
 .. [#userspace_nbdublk] https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk
 
 .. [#userspace_readme] https://github.com/ming1/ubdsrv/blob/master/README
-
-.. [#xiaoguang] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/