bpf: Remove unused declaration bpf_link_new_file()

Commit a3b80e10 ("bpf: Allocate ID for bpf_link")
removed the implementation but not the declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20230809140556.45836-1-yuehaibing@huawei.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>