sock_map: Kill sock_map_link_no_progs()

Now we can fold sock_map_link_no_progs() into sock_map_link()
and get rid of sock_map_link_no_progs().

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210331023237.41094-9-xiyou.wangcong@gmail.com