skbuff: remove unused skb_zcopy_abort function

skb_zcopy_abort() has no in-tree consumers, remove it.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>