Documentation: ublk: Drop Stefan Hajnoczi's message footnote

Sphinx reports unreferenced footnote warning pointing to ubd-control
message by Stefan Hajnoczi:

Documentation/block/ublk.rst:336: WARNING: Footnote [#] is not referenced. [ref.footnote]

Drop the footnote to squash above warning.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Fixes: 4093cb5 ("ublk_drv: add mechanism for supporting unprivileged ublk device")
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250122143456.68867-3-bagasdotme@gmail.com

Author: bagasme

Documentation/block/ublk.rst

@@ -333,6 +333,4 @@ References
 
 .. [#userspace_readme] https://github.com/ming1/ubdsrv/blob/master/README
 
-.. [#stefan] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/
-
 .. [#xiaoguang] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/