docs: Do not include from .../seqno-fence.c

There are no kerneldoc comments in drivers/dma-buf/seqno-fence.c, and it
appears there never have been.  Stop looking for comments there to
eliminate this warning:

    ./drivers/dma-buf/seqno-fence.c:1: warning: no structured comments found
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dma-buf.rst

@@ -139,9 +139,6 @@ DMA Fences
 Seqno Hardware Fences
 ~~~~~~~~~~~~~~~~~~~~~
 
-.. kernel-doc:: drivers/dma-buf/seqno-fence.c
-   :export:
-
 .. kernel-doc:: include/linux/seqno-fence.h
    :internal: