bpf: Remove unused vairable 'bpf_xdp_link_lops'

kernel/bpf/syscall.c:2263:34: warning: 'bpf_xdp_link_lops' defined but not used [-Wunused-const-variable=]
 static const struct bpf_link_ops bpf_xdp_link_lops;
                                  ^~~~~~~~~~~~~~~~~

commit 70ed506c

 ("bpf: Introduce pinnable bpf_link abstraction")
involded this unused variable, remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200326031613.19372-1-yuehaibing@huawei.com