doc/filters/libplacebo: remove outdated example

This example being first is now misleading because round-tripping
through hwdownload/hwupload is neither required nor recommended. Also,
the comment about avoiding format conversion is unnecessary because
`libplacebo` will now inherit the input frame format by default.

doc/filters.texi

@@ -16330,16 +16330,6 @@ This filter supports almost all of the above options as @ref{commands}.
 
 @subsection Examples
 @itemize
-@item
-Complete example for how to initialize the Vulkan device, upload frames to the
-GPU, perform filter conversion to yuv420p, and download frames back to the CPU
-for output. Note that in specific cases you can get around the need to perform
-format conversion by specifying the correct @code{format} filter option
-corresponding to the input frames.
-@example
-ffmpeg -i $INPUT -init_hw_device vulkan -vf hwupload,libplacebo=format=yuv420p,hwdownload,format=yuv420p $OUTPUT
-@end example
-
 @item
 Tone-map input to standard gamut BT.709 output:
 @example