xsk: Remove dangling function declaration from header file

xdp_umem_query() is dead for a long time, drop the declaration from
include/linux/netdevice.h

Fixes: c9b47cc1

 ("xsk: fix bug when trying to use both copy and zero-copy on one queue id")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20210303185636.18070-2-maciej.fijalkowski@intel.com