lib/xxhash: remove more unused xxh functions

lib/xxhash: remove more unused xxh functions

xxh32_reset() and xxh32_copy_state() are unused, and with those gone, the
xxh32_state struct is also unused.

xxh64_copy_state() is also unused.

Remove them all.

(Also fixes a comment above the xxh64_state that referred to it as
xxh32_state).

Link: https://lkml.kernel.org/r/20251024205120.454508-1-linux@treblig.org
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>