lib/xxhash: remove unused functions

xxh32_digest() and xxh32_update() were added in 2017 in the original
xxhash commit, but have remained unused.

Remove them.

Link: https://lkml.kernel.org/r/20250716133245.243363-1-linux@treblig.org


Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Dave Gilbert <linux@treblig.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>