xsk: Wipe out dead zero_copy_allocator declarations

zero_copy_allocator has been removed back when Bjorn Topel introduced
xsk_buff_pool. Remove references to it that were dangling in the tree.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20211210171511.11574-1-maciej.fijalkowski@intel.com