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 <[email protected]>
Fixes: 4093cb5 ("ublk_drv: add mechanism for supporting unprivileged ublk device")
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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/[email protected]/
-
 .. [#xiaoguang] https://lore.kernel.org/linux-block/[email protected]/